A method for using WebServices as a basis for implementing an application programming interface (API) of a high-level programming language between a service application and a client application is disclosed. Function parameters of an API function call represent a complex object structure (COS). The method includes serializing the COS into serialized payload data, encoding the serialized payload data to form a payload string that does not contain any XML-specific characters, and specifying the parameters of the API call in parts of an input message of a corresponding operation in a WSDL document. The method also includes transporting the payload string within the input message to the service application, decoding the payload string to yield the serialized payload data, and deserializing the serialized payload data to restore the COS representing the parameters of the API call. A system and computer program product for implementing the API on the basis of Web Services are also disclosed.