In collaboration with the Iowa State University Physics Education Research Group, we have investigated students' understanding of simple constant pressure calorimetry experiments involving physical processes and chemical processes. Physical processes involving calculations, for example calculating the specific heat of a metal by placing a hot piece of metal in cold water, does not pose much difficulty for students. Conceptual understanding of thermochemistry does pose a problem for students. Heat and thermal phenomena have been the subject of considerable investigation in the science education literature, but calorimetry has received little attention from science education researchers. We have developed a series of web-based computer simulations and guided inquiry tutorials to help student confront difficult topics in calorimetry. Our presentation will include a detailed analysis of student performance on solution calorimetry problems in an introductory university chemistry class for science and engineering majors. Data from written classroom exams and from several case studies will be discussed. Our findings reveal a number of learning difficulties. Students have difficulty with vocabulary terms involving thermochemistry, the law of conservation of energy, net changes in temperature of the solution (?T), and understanding the energy exchanged by a chemical reaction with the solution is due to bond breaking and bond forming during a chemical reaction.