The objectives of this study are: (a) Evaluate science major freshman students' ability to differentiate between heat energy and temperature, after having studied the topic of thermochemistry; (b) ascertain the degree to which students resist change from the caloric to the kinetic-molecular theory; (c) study the ability to differentiate between heat energy and temperature and its relationship to solving a problem of thermochemistry. Science major freshman students (n = 76) were tested on a three-item Test of Heat Energy and Temperature (THT) and a Thermochemistry Problem (TP). Both THT and the TP formed part of a semester exam given after the topic of thermochemistry had been studied in class. Results obtained show that even after having studied thermochemistry in an introductory freshman course, students have considerable difficulty in differentiating between heat energy and temperature. Although, all three items of THT required the understanding of kinetic-molecular theory, percentage of correct responses varied considerably: Item 1 (72%), Item 2 (64%), and Item 3 (51%). A correct response on Item 3 meant that different amounts of water in two different vessels at 100 degrees C, would have the same average kinetic energy, but the heat energy required to attain 100 degrees C was different--hence the confusion with heat as a substance (caloric theory). It was found that students who had good conceptual understanding on the TP did not perform significantly (X[squared] = 0.023) better on the THT. It is concluded that as science teachers we must be aware of the difficulties associated with the differentiation between heat energy and temperature and hence the need for teaching thermochemistry in order to facilitate conceptual understanding.