This study focused on the development of a two-tier multiple-choice diagnostic instrument, which was designed and then progressively modified, and implemented to assess students' understanding of solution chemistry concepts. The results of the study are derived from the responses of 756 Grade 11 students (age 16-17) from 14 different high schools who participated in the study. The final version of the instrument included a total of 13 items that addressed the six aspects of solution chemistry, and students' understandings in the test were challenged in multiple contexts with multiple modes and levels of representation. Cronbach alpha reliability coefficients for the content tier and both tiers of the test were found to be 0.697 and 0.748, respectively. Results indicated that a substantial number of students held an inadequate understanding of solution chemistry concepts. In addition, 21 alternative conceptions observed in more than 10% of the students were reported, along with discussion on possible sources of such conceptions.