Transmission electron microscopy (TEM) of organic concentrate, trace-element analysis and C-isotope analysis have been used in an attempt to determine the source of particular trace elements. It has been shown that Cu was occasionally concentrated in bacterial cell walls in the Toolebuc oil shales whereas V was mostly associated with the inorganic fraction. High concentrations of Mo may have their origin in N-fixing cyanobacteria which are probably major contributors to the organic matter in these oil shales. High concentrations of Ti in the Rundle-Condor oil shales are associated with the humic acid fraction. The latter are shown to be of a terrigenous-allochthonous source. High Ti-Cr concentrations in the Green River oil shale are also associated with the humic acids. Ti in higher than average concentrations is found in oil shales of lacustrine environment whereas high V content is typical of marine-deposited oil shales.