This paper deals with the comparative suffix -ška of the Mordvin languages Erzya and Moksha. Traditionally, the suffix has been regarded as the ending of the comparative case with the meaning ‘about the amount of X’. However, it has a rather limited range of uses in this function. Since the suffix is also employed as a derivational suffix that turn nouns into adjectives meaning ‘the size of X’, some researchers prefer to consider it primarily derivational rather than a case ending. Etymologically, the suffix most likely goes back to the lexical noun ška ‘time’ that still appears in Erzya, although other origins have also been suggested. In this paper, the morphological properties of the comparative suffix are surveyed as well as the syntactic functions of the comparative forms. Moreover, the path of grammaticalisation of the lexical noun into a suffix is outlined. It is shown that the suffix should indeed be considered a derivational ending in the contemporary languages. However, some aspects in its use suggest that there might be an on-going process possibly leading to the development of a case category.