This study investigates chemistry engineering students’ qualitative understanding of enthalpy and enthalpy change. Data was collected in 2006 at different times and in different educational settings with different qualitative and quantitative methods. A first version of two sets of categories of students’ qualitative understanding of enthalpy prior to instruction has been constructed from pre-questionnaire answers (n=64). Four students were selected for a deeper qualitative analysis. Data analysis is presently conducted and at the moment there are only preliminary results concerning enthalpy and enthalpy change available. One of the four students has difficulties discerning between the system property enthalpy and the energy transfer, i.e. enthalpy change. As the same pattern can be found among many students we wonder why enthalpy and enthalpy change is difficult for the students. The analysis of data continues in order to improve the definitions of the categories.