Smectite-illite reaction of sediment minerals in sedimentary basins is modelled as a one-step dehydration reaction. The water released from the mineral reaction can affect compaction process so as to retard porosity reduction and thus may cause overpressuring, especially in the reaction region. The coupled nonlinear model equations have been solved numerically, and the asymptotic analysis has been used to get the analytical solutions of the nonlinear model equations in some regions of most practical interests. The comparison of the obtained asymptotical solutions with the numerical results and real data shows very good agreement. ¿ 2000 American Geophysical Union