Heating the individual common abietadienoic acid methyl esters withp-toluene sulfonic acid in chloroform led to the formation of methyl abietadienoates other than the equilibrium mixture of methyl abietate, palustrate, and neoabietate that is usually formed by acid-catalyzed isomerization. Of these other methyl abietadienoates, the three principal constituents comprised 10%, 4% and 2% of the monomers; these compounds were isolated and identified as methyl 13β-abieta-7,9(11)-dien-18-oate, methyl 7,9(11)-abietadien-18-oate, and methyl 8,12-abietadien-18-oate, respectively.