One potential barrier to the development of educational hypermedia is the design of current hypermedia authoring tools (HATs) that unfortunately require higher knowledge and skills levels than possessed by most academics. Whilst the usability of hypermedia has been extensively researched, the usability of the tools required to build hypermedia has not. Learnability of HATs, an associated factor of usability, has been similarly neglected. Analysing contemporary approaches to the study of human computer interaction, this paper concludes that they do not support the kind of 'theory building' required to study and describe the learnability of HATs. Grounded theory (GT) is posited as an alternative approach, which if applied correctly can provide explanatory theory to inform HAT design. The paper describes the application of GT to two studies of the ease-of-learning of HATs. The first study uses quantitative and qualitative data to explore the experiences of 16 subjects learning to use HATs. In the second study, key HATs are demonstrated to a focus group of IT trainers to analyse their observations of users learning HATs. From these studies a causal model of learnability of HATs that is more detailed and complete than that offered by other contemporary theories of learnability was developed. The paper concludes that applying a GT approach can enhance HCI research through the development of explanatory, extensible and evolutionary theory to inform HAT design.