CO2 evolved from the carboxyl groups of amino acids upon heating with ninhydrin is measured manometrically in the Van Slyke-Neill gas machine. Of 24 amino acids tested all yielded 1 mole of CO2 save aspartic and cystine which yielded 2 and alanine, serine and tryptophane which evolved slightly less than 1. Configurations evolving CO2 with nin-hydrin are limited to those having a free NH2 group in the [alpha]-position adjacent to a carboxyl and certain acids with cyclic imino groups similar to the prolines. 0.05-2.0 mg. carboxyl CO3 may be detd. with an error of 1%.