The kinetics of the polymerisation of acrylonitrile inside wool fibres, leading to the internal deposition of polyacrylonitrile, have been investigated, using the ferrous ion-hydrogen peroxide technique of Lipson and Speakman. Both the rate of deposition of the polymer, and the saturation weight increases obtained, are increased by raising the temperature or the concentration of acrylonitrile, but both are virtually independent of the concentration of hydrogen peroxide over the range 0·001-0·01 per cent. The weight of polymer formed internally is directly proportional to the square root of the time of treatment up to weight increases of 40 per cent, but thereafter the rate of deposition continually decreases and saturation weight increases are found. This behaviour is characteristic of a diffusion-controlled process, and the experimental data are analysed in terms of an apparent diffusion coefficient, D. It is found that a value of D of 1 x 10-11 cm2/sec fits the data for loose Lincoln wool and is independent of the temperature and the concentration of monomer or catalyst. The data for Merino wool, however, correspond to a value of D of 1 x 10 - 12 cm2/sec. It is tentatively suggested that these apparent diffusion coefficients may correspond to the diffusion of acrylonitrile into the wool.