Logical Operations Test (BLOT) was developed for assessing the transition to formal operational thinking. BLOT is a 35 item multiple-choice test which examines all of the operations which comprise the logico-mathematical structure of formal operations in Piaget’s theory. The test was translated into Serbian and used in previously reported research. This work deals with two additional parallel versions of the Serbian BLOT. For each original BLOT item two more parallel items have been constructed by changing the item content and leaving the logical structure of the item the same. Sample consisted of 517 primary and secondary school students. Rasch analysis confirmed that the vast majority of items maintained invariance across at least two test versions: for 19 original items both parallel items maintained their invariance, for 14 items one of the parallel items had similar parameters and only 2 items did not remain invariant in the parallel tests.