The usability of Application Programming Interfaces (APIs) is one of the main factors defining the success of a software based framework. Research in the area of human computer interaction (HCI) currently mainly focuses on end-user usability and only little research has been done regarding the usability of APIs. In this paper, we present a methodology on how to use and combine HCI methods with the goal to evaluate the usability of APIs. The methodology consist of three phases - a heuristic evaluation, a developer workshop and interviews. We setup a case-study according to the methodology, in which we are evaluating the usability of a service-oriented framework API. The goal was to explore different HCI methods and compare the applicability of such methods to find usability problems in an API. The case-study combined qualitative and quantitative methods in order to investigate the usability and intuitiveness of the API itself. It allowed us to identify relevant problem areas for usability related issues that could be mapped to specific types of HCI methods. Examples for this are e.g. structural problems, which are identified mainly in inspection methods, while problems regarding errors and exception handling are mainly identified during the hands-on example part of the developer workshops conducted. The resulting problem areas allow us to develop a first classification of API related usability problems that are making the relevancy of usability issues for APIs more explicit and applicable.