The main goal of this study was to investigate micropetrographic characteristics of volcaniclastic deposits in vicinity of Sinj (in the southern part of the External Dinarides). The investigated sections are located near village Zelovo. Field investigation included analysis of the two sequences - SINJ and ZELOVO, where volcaniclastic deposits occur. Micropetrographic samples were performed in volcaniclastic and carbonate rocks. Volcaniclastics were differentiated into: vitriclastic, crystalloclastic and crystallovitriclastic rock types. Vitriclastic tuffs contain a large amount of shards - volcanic glass fragments that imply explosive volcanism of acidic or neutral magma composition. The crystalloclastic tuffs contain a large amount of quartz crystalloclasts that indicate acidic magmatism. Limestones are characterized by a presence of radiolarians and thin shells that commonly appear in the deeper, pelagic sedimentary environment, but green algae that inhabit the shallow parts of the marine environment also occur in the composition of limestone types. This indicates reworking of material from the shallow to the deep part of depositional environment. Volcaniclastic deposits are probably formed by similar mechanisms which include resedimentation of the pyroclastic material from the shallower to the deeper part of sedimentary basin by gravitational currents. In the undetermined contact with the deposits of the ZELOVO locality there are also basalts that we assumed to have been formed on the sea floor and had the effect on the limestones and caused the dolomitisation and silicification of the limestones.