The present invention is directed to labeled compounds, [2H1, 13C], [2H2, 13C] and [2H3, 13C]methyl aryl sulfones and [2H1, 13C], [2H2, 13C] and [2H3, 13C]methyl aryl sulfoxides, wherein the 13C methyl group attached to the sulfur of the sulfone or sulfoxide includes exactly one, two or three deuterium atoms and the aryl group is selected from the group consisting of 1-naphthyl, substituted 1-naphthyl, 2-naphthyl, substituted 2-naphthyl, and phenyl groups with the structure: wherein R1, R2, R3, R4 and R5 are each independently, hydrogen, a C1-C4 lower alkyl, a halogen, an amino group from the group consisting of NH2, NHR and NRR′ where R and R′ are each a C1-C4 lower alkyl, a phenyl, or an alkoxy group. The present invention is also directed to processes of preparing methyl aryl sulfones and methyl aryl sulfoxides.