S U M MARY: Source rock properties were determined on samples obtained from three modern African lakes--Tanganyika, Kivu and Edward--and on thermally immature material from Eocene Lake Uinta of Utah and Colorado (Green River Formation). The modern rift lakes represent largely clastic lacustrine systems, whereas Lake Uinta is a carbonate-dominated system. The sediments from both systems exhibit a wide range of organic enrichment: up to ~ 13 wt.~o TOC in the clastic lake samples and sometimes greater than 30 wt.~ TOC in the carbonate-dominated system. Total hydrocarbon generation potentials, as determined by 'Rock-Eval' pyrolyses, also indicate higher yields in the carbonate system (up to ~ 200 mg HC g-1 rock) as compared to the clastic systems (up to -,~ 90 mg HC g-1 rock). Differences in hydrocarbon yield are greater than can be explained by the organic content. They appear to be the combined result of mineral matrix effects and kerogen character. Elemental and visual analyses of isolated kerogen also support the idea of differences in organic character. The kerogen from the Green River Formation was used to define the Type I reference curve. The kerogen isolated from the clastic lakes appears intermediate between the Type I and Type II reference curves and contains varying and, in some cases, very significant proportions of Type III and/or residual (inert) organic material. In both cases, the principal product will be a high-wax crude oil. The primary differences in organic enrichment and character appear to be related to the relative quantity of allochthonous sedimentary material, both organic and inorganic.