The determination of rate constants of reactions of the type Na + RX = NaX + R by the diffusion flame method is critically investigated, both theoretically and experimentally. More precise methods than those used heretofore are developed for calculating rate constants from experimental data, viz. evaluation of the effective nozzle radius, a generalization of the Heller criterion, an improvement in the theory of the visibility limit, and allowance for wall reaction and halide consumption. These methods were applied to previous data on the reaction Na + C2H5Cl as well as to new data on the reaction of sodium with CH3Cl, CH3Br, CH3I, and C2H5Br.