The conversion of organic substances to petroleum in the sedimentary environment is discussed. The present available information is reviewed concerning probable source materials and the conditions of deposition. The significance of the porphyrin content of petroleum is reviewed. An outline of the thermodynamical and kinetic aspects of petroleum conversion is given. As a possible activating agent the radiations from radioactive materials in sediments are selected for discussion. A survey is made of the pertinent literature on known chemical reactions produced by alpha-particle bombardment. Possible mechanisms are mentioned whereby alpha particles may cause molecules to react chemically. Approximate calculations are made of the efficiency of conversion of organic material t petroleum by alpha-particle bombardment under certain hypothetical sedimentary conditions. These calculations show that the rate of conversion is highly dependent upon the relative association in the sediment between the organic material and the radioactive constituents. In a uniform, homogeneous sediment containing 2.5 per cent organic matter and having a concentration of 2.7×10^-6 gram of uranium and 8×10^-6 of thorium per gram, approximately 2/5 of the total organic content may be converted in 100 million years. Due to the uncertainty in the available data no definite conclusion may be made as yet as to the importance of sedimentary radioactivity in petroleum genesis.