diff --git a/app/controllers/cart_payments_controller.rb b/app/controllers/cart_payments_controller.rb
index 83724ac..4358d20 100644
--- a/app/controllers/cart_payments_controller.rb
+++ b/app/controllers/cart_payments_controller.rb
@@ -12,7 +12,7 @@ class CartPaymentsController < ApplicationController
     param :path, :cart_id, :integer, :required, 'Cart ID'
     param :body, :cart, :versionCart, :required, 'Cart'
     response :ok, 'Success', :readTransaction
-    response :not_found, 'No cart with that ID'
+    response :not_found, 'No cart with that ID or it is expired'
     response :conflict, 'The user\'s balance is too low'
     response :precondition_required, 'The cart is stale'
     response :gone, 'The cart payment is already being processed'
diff --git a/app/controllers/carts_controller.rb b/app/controllers/carts_controller.rb
index 8f1cacb..e4a8c1b 100644
--- a/app/controllers/carts_controller.rb
+++ b/app/controllers/carts_controller.rb
@@ -48,6 +48,9 @@ class CartsController < ApplicationController
     property :id, :integer, :optional, 'Cart ID'
     property :user_id, :integer, :optional, 'User ID'
     property :total_price, :integer, :optional, 'Total Cart price'
+    property :expires_at, :date_time, :optional,
+             'Date and time at which the cart expires, making it read-only, '\
+             'not payable for, and releasing product reservations'
     property :lock_version, :integer, :optional, 'Cart version'
     property :cart_items, :array, :optional, 'Cart Items',
       'items' => {'$ref' => 'readCartItem'}
@@ -103,6 +106,7 @@ def update
     if cart.save
       render json: cart, status: :ok, location: cart
     else
+      cart.reload
       render json: cart, status: :conflict, location: cart
     end
   end
diff --git a/app/interactors/resolve_cart_and_user.rb b/app/interactors/resolve_cart_and_user.rb
index 2db1651..3c77505 100644
--- a/app/interactors/resolve_cart_and_user.rb
+++ b/app/interactors/resolve_cart_and_user.rb
@@ -3,7 +3,7 @@ class ResolveCartAndUser
 
   def call
     begin
-      context.cart = Cart.find_by_id!(context.cart_id)
+      context.cart = Cart.unexpired.find_by_id!(context.cart_id)
     rescue ActiveRecord::RecordNotFound
       context.fail!(message: 'generic.not_found')
     end
diff --git a/app/models/cart.rb b/app/models/cart.rb
index 696d333..fb62f15 100644
--- a/app/models/cart.rb
+++ b/app/models/cart.rb
@@ -1,8 +1,34 @@
 class Cart < ActiveRecord::Base
+  EXPIRE_AFTER = 300 # seconds
+
   belongs_to :user
   has_many :cart_items, dependent: :destroy, autosave: true
 
+  validate :ensure_unexpired
+
+  scope :unexpired, -> { where unexpired_condition }
+
+  def self.unexpired_condition
+    arel_table[:updated_at].gteq(EXPIRE_AFTER.seconds.ago)
+  end
+
   def total_price
     cart_items.map(&:total_price).reduce(:+)
   end
+
+  def expires_at
+    (updated_at || DateTime.now) + EXPIRE_AFTER.seconds
+  end
+
+  def expired?
+    DateTime.now > expires_at
+  end
+
+  private
+
+  def ensure_unexpired
+    if expired?
+      errors.add :expires_at, "can't be in the past"
+    end
+  end
 end
diff --git a/app/models/cart_item.rb b/app/models/cart_item.rb
index f276047..8538bec 100644
--- a/app/models/cart_item.rb
+++ b/app/models/cart_item.rb
@@ -5,6 +5,8 @@ class CartItem < ActiveRecord::Base
 
   validate :enough_items_present_in_pricing
 
+  scope :unexpired, -> { joins(:cart).where(Cart.unexpired_condition) }
+
   def product_name
     product.try(:name) || ''
   end
diff --git a/app/models/pricing.rb b/app/models/pricing.rb
index 850054d..af98e77 100644
--- a/app/models/pricing.rb
+++ b/app/models/pricing.rb
@@ -6,6 +6,6 @@ class Pricing < ActiveRecord::Base
   validates :price, numericality: { only_integer: true }
 
   def available_quantity
-    quantity - cart_items.sum(:quantity)
+    quantity - cart_items.unexpired.sum(:quantity)
   end
 end
diff --git a/app/representers/cart_representer.rb b/app/representers/cart_representer.rb
index 17c0d25..c784f6a 100644
--- a/app/representers/cart_representer.rb
+++ b/app/representers/cart_representer.rb
@@ -4,6 +4,7 @@ class CartRepresenter < ApplicationDecorator
   property :lock_version, type: Integer
   property :user_id, type: Integer
   property :total_price, writeable: false, type: Integer
+  property :expires_at, writeable: false, type: Date
 
   collection(
     :cart_items,