Field desorption mass spectrometry was shown to be a valid technique for determining the molecular weights of hemins and hemin esters, as well as of porphyrins. The observed base peaks of ligand-free protoheme IX, protoheme IX dimethyl ester, and protoporphyrin IX dimethyl ester correspond well to the molecular weights of these compounds and the base peak for hematoporphyrin corresponds to the molecular weight of this porphyrin minus two molecules of water. The technique was employed to confirm the molecular weights of heme alpha, heme alpha dimethyl ester, and acetyl heme alpha dimethyl ester. Heme alpha dimethyl ester was prepared by reaction of heme alpha with trimethyloxonium tetrafluoroborate and purified by high-pressure liquid chromatography. The isolated product was converted to the acetylated derivative by reaction with acetic anhydride and was subsequently purified by high-pressure liquid chromatography. A field desorption spectrum of heme alpha shows a base peak at 582 which is in agreement with previous deductions of the structure of this prosthetic group. Base peaks of the heme alpha ester and its acetylated derivative demonstrate that the two carboxyl groups have been methylated and the single hydroxyl group has been acetylated without further alteration of the molecule.