On the basis of three “irregular” forms of the 2nd sigmatic aorist in the Old Church Slavonic Codices Zographensis and Marianus the contribution reconstructs the prehistory of the mor phological category as a process of sigmatization of the simple or thematic aorist and a perfor mation of the 1st sigmatic aorist, starting in both cases from the 2nd–3rd singular and realized by adding the sigmatic endings of the 1st sigmatic aorist on the one hand to the personal stems of the 2nd singular – 2nd plural and the personal forms of the 1st singular and 3rd plural of the thematic aorist, on the other hand to the personal stems of the 2nd–3rd singular of the 1st sig matic aorist, where the formation was then generalized in the whole paradigm.