To assess students' conceptual understanding levels and diagnose alternative frameworks of the electrolyte concept, a measurement instrument was developed using the Rasch model. This paper reports the use of the measurement instrument to assess 559 students from grade 10 to grade 12 in two cities. The results provided both diagnostic and summative information about students' conceptual understanding, suggesting that this measurement instrument had a certain validity. The results also demonstrated that Chinese mainland senior students' understanding improved with increasing grade level, but that many alternative frameworks were entertained by students at each level.