This paper reports on work which is an extension of the work previously reported in Boo (1995) which involved A-level chemistry students. The present paper reports on investigative studies conducted with university undergraduates and graduates in chemistry to examine their ability to reason scientifically when confronted with a range of chemical phenomena. The chief data collection instrument is the clinical interview. Five familiar chemical reactions were used as foci for discussion in the interviews; during which interviewees were asked to make predictions about the type of change expected, including the overall energy change involved. They were also asked to describe how they thought the change proceeded at the microscopic level as well as to explain why they thought the change took place at all.