The purpose of this study is to determine the item quality of acid-base assessment instruments on the sub-microscopic level using the Rasch model. This is achieved by identifying logit values and Rasch reliability components for each item (Multiple Choice Questions/MCQs). The method used was a quantitative approach. The data was collected by using the instrument which consisted of 15 items of acid-base assessment at the sub-microscopic level. Analysis of the data was conducted by using the Rasch model to probe the reliability of the item. Based on the data, the item reliability (0.99) of the assessment instrument on chemistry material at the sub-microscopic level can be stated as a measurement tool of the chemistry concept understanding especially on acid-base material. In addition, MCQs based at the sub-microscopic level was an alternative way of detecting student understanding in chemistry material. Therefore, the concept of justification of items with the Rasch model can be used as a reference for improving student understanding, quality of teaching and learning, and guidelines in designing chemistry learning strategies based on an analysis of student understanding. Key words: Rasch model, sub-microscopic level, multiple choice test, acid-base material.