Abstract: Method for quantifying the amounts of oxidized sulfhydryl amino acids, particularly cysteine sulfinic acid, cysteic acid, homocysteine sulfinic acid and homocysteic acid, for example, in biological samples are provided. The oxidized sulfhydryl amino acids are measured by gas chromatography/mass spectrometry after derivatization, preferably using stable isotope internal standards. Deviation from normal individuals' levels of oxidized sulfhydryl amino acids in serum are indicative of certain neuropsychiatric disorders, the biochemical origin of which can be diagnosed using methods disclosed herein, particularly when relative levels of these and/or other sulfur amino acids are measured.