In the fractions of the methyl esters of urinary organic acids seventeen N-methylated or N,N-dimethylated amino acid methyl esters are identified by gas chromatography-mass spectrometry. It is shown for twelve amino acids that their amino group reacts with diazomethane to form these derivatives. Using deuterated reagents, in particular deuterated diazomethane, in the sample preparation procedure during the organic acid analysis, it is shown that the N-methylated and N,N-dimethylated amino acids are artifacts from diazomethane and are not biochemical N-methylation products.