Rate coefficients for the gas-phase reactions of 1,3,5-trimethylbenzene (TMB) and rn-xylene with the OH radical relative to 10 reference compounds were measured yielding (57.3 +/- 5.3) x 10(-12) and (22.0 +/- 2.7) x 10(-12) cm(3) molecule(-1) s(-1), respectively (uncertainties are 2 sigma). These uncertainties arise from a combination of the uncertainty of the reference rate coefficients and experimental errors; thus, the average uncertainty for the rate coefficients of the 10 reference compounds must be less than or equal to the scatter observed in this study. For 16 and 21 measurements with TMB and m-xylene, the observed scatter was +/-10% and +/-12%, respectively. The average of the uncertainties is significantly smaller than the uncertainties that are frequently assumed or recommended, which range from +/-15 to 35%. Absolute rate coefficients for the reaction of TMB and m-xylene with ozone are also reported. Relative rate coefficients for reaction with OH for di-n-butyl ether, methylcyclohexane, cyclopentane, toluene, and cyclohexane of 28.9 +/- 2.3, 9.4 +/- 0.6, 4.8 +/- 0.6, 5.5 +/- 0.5, and 7.2 +/- 0.6 (x10(-12) cm(3) molecule(-1) s(-1)) were also measured (uncertainties indicate the whole range of scatter).