Laser flash photolysis (Nd:YAG, 35 ps, 266 nm, 10 mJ; KrF excimer, 349 nm, 12 ns, 70 mJ; XeCl excimer, 308 nm, 17 ns, 50 mJ) of methyl substituted phenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected directly, and their decay was analyzed to yield the observed rate constant k(OBS). The observed rate constant is the sum of k(R) + k(ISC) where k(R) is the absolute rate constant of rearrangement to an azirine and k(ISC) is the absolute rate constant of intersystem crossing. Values of k(OBS) were measured as a function of temperature and dissected assuming that k(ISC) is temperature-independent, It was shown that k(ISC) is independent of temperature for 2,6-dimethyl- and 2,3,6-trimethylphenyl singlet nitrenes. The barriers to rearrangement of singlet phenylnitrene, p-methylphenylnitrene, o-methylphenylnitrene, 2,6-dimethylphenylnitrene, and 2,4,6-trimethylphenylnitrene in hydrocarbon solvents are found to be 5.6, 5.8, 5.3, 7.0, and 7.3 +/- 0.4 kcal/mol, respectively. The data demonstrate that two ortho methyl substituents retard cyclization but that a single ortho or para substituent does not. It is concluded that the rate of retardation provided by the methyl groups has a steric origin, as predicted by Karney and Borden (J. Am. Chem. Sec. 1997, 119, 3347). These authors predict that ortho methylation raises the barrier to cyclization by 1-2 kcal/mol, in excellent agreement with our results. Our results are also consistent with the work of Sundberg et al. (J. Am. Chem. Sec. 1972, 94, 513), which demonstrated that singlet 2-methylphenylnitrene cyclizes away from the ortho methyl substituent. Methyl substitution is found to increase the absolute rate constant of intersystem crossing.