This paper studies the effect of alkaline oil shale ash on oil shale thermal decomposition products during oil shale and ash co-retorting. Experiments with and without ash were carried out in a standard laboratory-scale retort (Fischer Assay) to determine relative yields of retorting products and sulfur distribution in them. Oil shales compared in this study were kukersite and Dictyonema shales from Estonia, Jordanian El-Lajjun oil shale and Saveljev oil shale from Russia. Oil shales were primarily selected due their difference in sulfur composition. Ash for co-retorting experiments was prepared front kukersite oil shale. The ash contains about 57.5 wt.% alkaline compounds CaO and MgO.