Background: The development of two-tier multiple-choice questions has permitted the diagnosis of students' understanding on various topics in the sciences as well as helped to ascertain the alternative conceptions they have. A limitation of two-tier diagnostic instruments that has been noted in the literature, but which has not been systematically explored, is that some questions can have a diagonal response. This can allow students to select a second-tier response based on cues from a particular response in the first tier, and it can limit the analytical utility of the diagnostic instrument to some extent. Purpose: The study explores an enhancement of the traditional two-tier diagnostic instrument to address diagonal response. In the format proposed, there are dual sections for both the answer and reason tiers. The answer tier is in dichotomous form and for each of the dichotomous responses, there are separate polytomous responses in the reason tier. Sample: There were 99 students of grade 10 level who participated in this study. Design and Method: The research design follows the usual approach adopted for the development of diagnostic instruments but with some modifications. The topic of electrochemical cells was chosen to evaluate the effectiveness of the enhanced format of the two-tier instrument. Results: It was found that with the enhanced format, the issue of diagonal response has been satisfactorily addressed. Also, a situation where students get the answer wrong but the reason correct, which is sometimes encountered in two-tier questions, does not arise with this format. The instrument has also been able to shed some light on the students' understanding of electrochemical cells and their alternative conceptions. Conclusions: The enhanced format of the two-tier instrument proposed in this study can also be used for the study of alternative conceptions in science education.