This study explored undergraduate students' understanding of the chemistry topic of thermodynamics using a 4-tier diagnostic instrument, comprising 30 questions, and follow-up interviews. An additional objective of the study was to assess the utility of the 4-tier instrument for use in studies on alternative conceptions (ACs) as there has been no study done on it since its introduction in the literature in the year 2010. A total of 296 students majoring in Chemistry at a university in Singapore participated in this study--88 students in the preliminary study, 102 students in the pilot study and 106 students in the main study. This article reports on the results obtained with students in the main study; their age ranges from 20 to 22 years. Comprising answer and reason tiers plus associated confidence ratings, the 4-tier diagnostic instrument enabled the eliciting of 34 ACs harbored by the undergraduates as well as the strengths of these ACs. Of concern to note is that even for questions which were answered correctly, the mean confidence was not very high. The results of this study reiterate the point that thermodynamics is a topic fraught with conceptual difficulties and ACs. Based on the results from this study, the potential of the 4-tier test for AC studies is further underscored. Some implications of the study are discussed. (Contains 5 tables.)