A computer program, DEMOS (for Decomposition Mode Solution), has been written to estimate the chemical concentrations and rate constants in a computer model of a multienzyme system such that it accurately reproduces the instantaneous kinetic state of the system. The entire system is defined in the BIOSSIM simulation language as a set of chemical equations with associated concentrations and rate constants, and the language generates all necessary FORTRAN subroutines automatically. DEMOS estimates the parameter values in a metabolic model by formal optimization subject to user-supplied constraints and weighting. The definitions of the kinetic state to be matched by the computer model and the unknown concentrations to be estimated are expressed in the DEMOS input by a simple and natural syntax. The program includes provision for input, output, graphical display, and a simple but highly flexible procedure for interactively altering the values of rate constants or concentrations and redefining the optimization problem. DEMOS is a "friendly" program which does not require computer expertise, and it is inexpensive to use.