Pyrolysis-Gas Chromatography and Pyrolysis-Gas Chromatography-Mass Spectrometric studies were carried out on smectite/kerogen and illite/kerogen mixtures. Using homoionic cation-exchanged clays the catalyzed production of hydrocarbons depends on the ionic potential (ionic radius/ionic charge) of the exchange cation, and decreases in the order Mg⁺⁺>Ca⁺⁺>Na⁺>K⁺. This effect is related to the Bronsted acidity developed as the exchangeable cations cause dissociation of solvating water molecules in the interlayers of smectite and on external surfaces of illite crystallites. GC-MS characterization of the hydrocarbon pyrolysates indicates increasing isoalkane/n-alkane ratios (branching ratios), suggesting the importance of carbonium ion mechanisms induced by the Bronsted acidity.