We present the main petrographic and geochemical features of the Lower to Middle Eocene turbiditic sandstones from the northwestern portion of the External Dinaride flysch basin. Sampled areas cover SW Slovenia (Vipava and Brkini Basins) and the Istrian Peninsula (Trieste-Koper and Pazin Basins). Framework constituents of the lithic arenites reveal low-grade metamorphic, acidic plutonic, and to a lesser extent, mafic volcanic and ultrabasic sediment sources, with evidence for a small degree of sediment recycling as well. Among the processes that commonly influence sediment compositions, weathering in the source and sorting were probably negligible, but carbonate contribution of detrital or intrabasinal origin diluted the siliciclastic portions to various degrees. Main and trace element compositional data agree well with petrography and clearly indicate the predominance of felsic, crustal source lithologies. Exposed mafic-ultramafic source units were volumetrically less important. From the Early Paleogene, extensive sediment mixing occurred in front of the Dinaride orogenic thrust wedge, with the components derived from different Dinaride units of felsic crystalline basement, platform carbonates and ophiolite. In the Eocene, a likely source of the mafic-ultramafic detritus was the Jurassic ophiolitic melange in the NE Dinarides.