A protocol converter mediates the communication between implementations of different protocols, enabling them to achieve some form of useful interaction. The problem of deriving a protocol converter from specifications of the protocols and a desired service can be viewed as the problem of finding the 'quotient' of two specifications. We define a class of finite-state specifications and present an algorithm for solving 'quotient' problems for the class. The algorithm is applied to an example conversion problem. We also discuss its application in the context of layered network architectures.