Kinetic profiles have been determined for the pyridine catalysed reaction of Corsican pine (Pinusnigra) sapwood, with an homologous series of linear chain carboxylic acid anhydrides. A model based upon percolation theory has been developed to explain the observed reaction profiles. The model is described, and the results from this are compared with experimental data. By considering the combined effects of diffusion of reagent on a porous network and rate of reaction at sites within the network, the observed experimental kinetic curves have been reproduced. It has been found that the reaction profiles can be reproduced by varying one parameter in the model, which represents the rate of reaction of molecules with active sites located in the network. The kinetics of the reaction of acetic anhydride with whole wood is diffusion controlled throughout. This indicates that wood chemically modified by this reagent will be reacted to different extents, depending upon the local density of the substrate.