When two goods are completely interchangeable, they are perfectsubstitutes. A perfect substitute is a product or service that can be used in exactly the same way as the good or service it replaces. This is where the utility of the product or service is pretty much identical. An example would be Coke or Pepsi, BP petroleum or Exxon petroleum, etc. A ‘substitute’ or ‘substitute good’ in economics and consumer theory is a product or service that aconsumer sees as the same or similar to another product.A substitute can be perfect or imperfect depending on whetherthe substitute completely or partially satisfies the consumer. If aconsumer sees a difference between these brands, he may seePepsi as an imperfect substitute for Coke, even if the economists might consider them perfect substitutes.