All the stationary points on the So surface of H2CS and their quadratic force fields have been determined by correlated methods using large multiply polarized basis sets. Complete quartic force fields of all stationary points and reaction paths connecting each pair of minima through a first-order saddle point have been obtained at the MP2 level using smaller polarized basis sets. These data allow the computation of reliable isomerization and fragmentation rates by semiclassical expressions which take into account tunneling as well as curvature and mode-mode couplings. Further insight into the characteristics of the different reaction channels has been gained from the parameters of the so-called reaction path Hamiltonian (RPH) both in adiabatic and diabatic representations. The bookkeeping and processing of the large body of data involved in this study has been possible through the development of a package which is able to perform anharmonic and reaction path computations. The results are discussed with special reference to the H2CO system and to the interplay of potential, kinetic, and statistic effects in determining reaction mechanisms and rates.