Secondary porosity, formed by the dissolution of both carbonate and silicate minerals, especially K-feldspars, is widely developed in sandstones of the Frio Formation (Oligocene) in the Texas Gulf Coast. CO2 produced by decarboxylation of organic matter is commonly suggested as the acid required for dissolution. Material balance calculations indicate that CO2 produced by decarboxylation of organic matter in Frio Formation shales can account for a regional average of only 1% or 2% secondary porosity in Frio Formation sandstones, yet point-count data indicate an average of 10% secondary porosity. Long-distance fluid transport (many kilometres) and/or other mechanisms of acid generation should, therefore, be considered. Carbon isotopic data on dissolved inorganic carbon in formation water, CO2 in produced natural gas, and carbonate cements indicate that CO2 produced by decarboxylation had a minor impact on these carbon reservoirs. The reaction kerogen + water → methane + carbon dioxide can explain the isotopic data, but alone it is insufficient to account for all the secondary porosity unless long-distance material transport is involved.