A continuous flow laboratory reactor designed for kinetic studies of gas-solid reactions at low pressures is described. Its application is illustrated by the low pressure chemical vapour deposition (LPCVD) of polycrystalline silicon. Mixing in the reactor is based on molecular diffusion. On-line monitoring of the deposition rate is performed with a microbalance. A mathematical model to analyse the experimental data is presented. Special attention is paid to the effects of irreducible concentration gradients of gas phase intermediates. By taking these effects quantitatively into account it is possible to obtain rate coefficients which are intrinsic, i.e. only determined by chemical phenomena.