A mass spectrometer equipped with a heated inlet system operated at 200 degrees C shows remarkable ability to differentiate among the various very similar structures for 13 of the 14 well characterized resin acids. Some isomerization of the most heat- sensitive compounds does occur but causes no difficulty in the method. An analysis of resin acids is obtained in about 2 hours, using an internal standard technique and a computer for solution of 13 or more simultaneous equations. The method has been applied to a 12 component text mixture and to several types of rosin and modified rosin. It now gives an almost complete picture of the resin acid portion of rosin. Completion of this picture depends on obtaining for calibration of the mass spectrometer several resin acids not yet isolated from rosin.