Research shows that students have problems understanding thermodynamic concepts and that a gap exists at the tertiary level related to more specific chemistry concepts such as enthalpy. Therefore, the aim of this study is to construct undergraduate students' conceptions of enthalpy, its change and related concepts. Three explorative small-scale studies were conducted at two Swedish universities. Questionnaires, exam questions, hand-ins and interviews covered a range of issues from chemical thermodynamics in general to specific questions about enthalpy and its change, internal energy and its change, heat and work. Data were analysed iteratively and qualitative categories were constructed (F1-F2, F4-F9). The underlying conceptions indicate that constant pressure is explicitly expressed but disregarded as the answer is given (F1), that work is described as mechanical work (F2), that enthalpy is used as a form of energy (F4), and that enthalpy is used for enthalpy change and "vice versa" (F5). The logical conceptions indicate that molar enthalpy determines the heat given off by a reaction and not the path taken (F6), that constant pressure/constant volume and the definition of enthalpy change are problematic (F7), that students argue for the case when ?H = ?U instead of ?H = q (F8), and that there are different ways to interpret the given tasks (F9). This study offers insight into the way students use enthalpy and its change when arguing and solving qualitative tasks. How the categories may be used as well as other implications for teaching and research are addressed in this paper.