The organic matter associated with two Mississippi Valley-type deposits (Gays River, Nova Scotia, and Shullsburg, southwestern Wisconsin) has been characterized by petrographic, isotopic, spectrophotometric, gas chromatographic, and mass spectral methods. Comparisons were made of distributions of organic species identified from unmineralized and mineralized samples. The primary variations in the distributions of organic species isolated from the two deposits reflect variations in host-rock paleoecology, facies, and geologic age. Secondary effects were microbial degradation, thermal maturation, and mineralization. Mineralization in both deposits caused an increase in the concentrations of low molecular weight hydrocarbons.The genetic model tested in this study presumed that mineralizing brines evolved from sedimentary basins and migrated to the ore host rocks. This model implies that an epigenetic generation of organic matter should occur in the deposits. In the Gays River deposit, the probable aquifer for the mineralizing solutions contained an organic distribution markedly different from the indigenous organic matter in the host reef and can be interpreted as a second, epigenetic organic fraction added to the deposit. Evidence was not observed for an epigenetic organic generation in the Shullsburg deposit. The thermal maturity of the organic matter in the Wisconsin district supports a previous estimate of 0.25 m.y. to form a major orebody in the district.