The purpose of this study was to examine the changes in 19 preservice chemistry teachers’ understandings of the structure of matter, including the aspects of the physical states of matter, the physical composition of matter, and the chemical composition of matter, before, immediately after, and months after they received a specific instruction. The one-group pre, post, and delayed posttest design was used, and participants’ understandings before, immediately after, and months after the instruction were assessed using the same “three part particulate drawing” classification question constructed by Sanger (2000). Collected data were analyzed according to both the number of scientifically appropriate classifications, and the types and nature of scientifically inappropriate classifications made by preservice teachers. The results of these two analyses were quite parallel to each other and showed that this specific instruction promoted the development of participants’ scientific understandings of the structure of matter. It should be noticed that while the effect of the instruction appeared extremely positive based on the results of the statistical analyses which solely compared the number of scientifically appropriate classifications, it was reflected more accurately after the participants’ scientifically inappropriate classifications of the structure of matter were analyzed more thoroughly. It was also found that although some scientifically inappropriate classifications were changed to scientifically appropriate ones following the instruction, some of them reverted back to their initial status months after the instruction.