We develop an analytical Knudsen layer model at the ablative surface in gas flow: this model takes into account the temperature and velocity gradients in the bulk gas and the rebounding of gas molecules by the ablative wall back into the gas region. in addition, this model uses a bimodal velocity distribution function which preserves the laws of conservation of mass, momentum and energy within the Knudsen layer and converges to the Chapman-Enskog velocity distribution function at the outer boundary of the Knudsen layer. This model enables us to obtain the boundary conditions at the ablative surface in gas flow for arbitrary condensation and accommodation coefficients, which can be used for computational fluid dynamics simulation of ablation avoiding "micro" modelling of the evaporation process at the mean free path scale. (C) 2009 Elsevier Ltd. All rights reserved.