Application programming interfaces (APIs) are the interfaces to existing code structures, such as widgets, frameworks, or toolkits. Therefore, they very much do have an impact on the quality of the resulting system. So ensuring that developers can make the most out of them is an important challenge. However standard usability evaluation methods as known from HCI have limitations in grasping the interaction between developer and API – the GUI, which makes this interaction obvious, is missing. In this paper we present a longitudinal approach using concept maps and a question diary to make this interaction visible and study the usability of an API over time. Keywords API usability, longitudinal evaluation, concept maps. ACM Classification Keywords H5.2. [Information Interfaces and Presentation]: User Interfaces. General Terms Experimentation, Human Factors.