The present work investigates the effect of adding small amounts of humidity on the inhibition of natural gas-air flames. The inhibition is quantified by measuring and calculating the laminar burning velocities (S-u) of premixed flames from a C-1-C-2 mechanism. The experimental apparatus consists of a Mache-Hebra burner, equipped with flow controllers and air purification system. Steam is generated by injecting water into a preheated natural gas-air stream, by means of a syringe pump. The burning velocities are determined experimentally from the schlieren photography using the total flame area. The results indicate decreasing burning velocities with increasing steam concentration, demonstrating the importance of thermal capacity of water vapour on slowing down the flame propagation. There is no indication of flame acceleration due to kinetic considerations, even when the flames are doped with minute moisture loadings. It is shown in the calculations that the laminar burning velocity depends strongly on the number of grid points, and so a scaling relationship is developed for adjusting the computed values of S-u. The kinetic model predicts closely the experimental results, but the agreement between the experimental and numerical data is better at lower temperatures. The relationship between S-u and the concentration of the added water vapour, as calculated from the model, is linear. For the natural gas considered in this work, the laminar burning velocity at the atmospheric pressure decreases by 1.81 cm s(-1) at 150 degrees C for each percentage point of humidity present in the gas mixture, and by 1.18 cm s(-1) at 20 degrees C.