The use of implicit methods for numerically solving stiff systems of differential equa,. tions requires the solution of systems of nonlinear equations. Normally these are solved by a Newtontype process, in which we have to solve systems of linea.r equations. The Jacobian of the derivative function determines the structure of the matrices of these linear systems. Since it often occurs that the components of the derivative function only depend on a small number of variables, the system can be considerably sparse. Hence, it can be worth the effort to use a sparse matrix solver instead of a dense LU-decomposition. This paper reports on experiences with the direct sparse matrix solvers MA28 by Duff (1), Yl2M by Zlatev et al. [2) a.nd one special-purpose matrix solver, all embedded in the parallel ODE solver PSODE by Sommeijer [3). The authors a.re grateful to B. P. Sommeijer for his careful reading of the manuscript and for suggesting severe.I improvements. The research reported in this pa.per was supported by STW (Dutch Foundation for Technical Sciences). 43 44 J. J. B. DE SWARl' AND J. G. BLOM numerical solution of the linear systeDlS is given in Section 4. Finally, in Section 5, numerical experiments give insight into the beha.vior of these matrix solvers.