A series of di(naphthyl)carbenes (DNCs) having methyl groups on aromatic rings were generated by photolysis of the corresponding diazo precursors and studied not only by product analysis, but also by spectroscopic means. "Parent" triplet alpha-DNC was shown to have a half-life of 70 ms, which is some 30 times larger than that of triplet diphenylcarbene ((DPC)-D-3), whereas parent triplet beta-DNC was 2 orders of magnitude shorter-lived than the alpha-isomer. The lifetimes of triplet DNCs were significantly increased by introducing methyl groups near the carbene center. Thus, (3)alpha-DNC, which has four methyl groups at 2,2',4,4'-positions, was shown to have a half-life of 100 ms, and the replacement of the two methyl groups at the 2,2'-positions of this carbene with tri(deuterio)methyl groups resulted in an increase of the lifetime by approximately 3 times by quenching the intramolecular H transfer from the methyl groups to the carbene center leading to o-quinoid compounds. The results are discussed in terms of the counteracting effects of electronic properties stabilizing the singlet state and steric factors favoring the triplet and compared with similar studies with (DPCs)-D-3.