This work aimed at comparing and modelling the kinetics of the free radical grafting of maleic anhydride onto polypropylene (PP) in the solid state with and without supercritical CO2 (scCO(2)). The kinetics of the decomposition of dicumyl peroxide (DCP) in scCO(2) and in N-2 was first studied. The one in scCO(2) could be described by k(d)=1.32 x 10(14) xexp(-139.3kJ mol(-1)/RT). Without scCO(2) the overall grafting reaction rate was controlled, at least partly, by diffusion whereas with scCO(2) it was controlled mainly by the grafting reaction itself. Based on the experimental results, the following general kinetic model for the free radical grafting in scCO(2) was developed: Rg =3.35 x 10(12) xexp(-114.5 kJ mol(-1)/RT) [1] (0.99) [M](0.97) (mol L-1 s(-1)). (c) 2007 Elsevier Ltd. All rights reserved.