As is well known, PIE possessed several distinct sigmatic formations with modal or future-like semantics. The paper deals with two sigmatic formations which must be reconstructed for PIE and obviously possessed a similar semantic value. First: a full grade - si̯e/o-formation which is attested in Indo-Iranian, Continental Celtic and Balto-Slavonic; and second, an athematic - s-formation which is attested in Italic and in the Eastern branch of Baltic. The diverging morphology of these formations implies that they originally also differed in their semantics. The problem is that both formations are reflected as simple future tense in all daughterlanguages which preserved them. However, it seems possible to detect the original semantic difference between these formations by using the evidence of the only IE branch which preserved both formations side by side, i.e. Baltic. The paper investigates the morphology of the sigmatic future tense in dialects of Lithuanian and Latvian and shows that for the common prehistory of East Baltic dialects a secondary conflation of originally independent PIE formations—- si̯e/o-formation and - s-formation—in one single paradigm must be assumed. The particular distribution of both formations within the unified paradigm of Proto-East-Baltic makes it possible to obtain information on the lost semantic difference between them. Possible traces of the - si̯e/o-formation in the only recorded West Baltic language, Old Prussian, seem to confirm the conclusions drawn on the basis of the East Baltic evidence.