The Rayleigh scattering technique has been applied to a V-shaped laminar flame in order to investigate the effect of reactant temperature on the thickness and thermal structure of an H-2-enriched laminar methane-air flame. A systematic comparison of experimental and numerical results obtained with the GRI3.0 chemical mechanism is provided. First, the effects of reactant temperature on the pure methane-air flame are presented. A decrease in flame thickness and higher temperature gradients in the flame front are observed, and the maximum temperature gradient is shifted toward lower progress variable values. The preheat zone is substantially modified by the reactant preheating. Globally, the numerical predictions are in good agreement with the experimental results and validate the measurements. In addition, the effects of H-2 enrichment on the thermal methane/air flame thickness are presented for a reactant temperature of 300 K. We show that the main role of H-2 enrichment is to increase the temperature gradient and to shift toward lower values of the thermal progress variable c the location of the maximum temperature gradient. The reactivity of the mixture is strongly increased. At constant equivalence ratio, the burnt gas temperature is not modified by the H-2 enrichment, and the analysis can be done either in c or T space. Finally, the effect of reactant preheating on H-2/CH4/air flames (40% H-2 dilution in volume) is investigated. The flame thickness is found to be lowered when reactant temperature is increased, but unlike the preheated methane-air flame, the localization of the maximum temperature gradient is not found to be markedly shifted. The reactivity of the mixture is rather controlled by the H radicals produced by H-2 dilution than the reactant preheating.