Fast efficient numerical programs for determining the equilibrium composition of large chemical systems subject to generalized linear constraints are needed for chemical kinetic calculations involving both the conventional local thermodynamic equilibrium (LTE) and the more general rate-controlled constrained-equilibrium RCCE assumptions. For this purpose two callable subroutines based on the well known NASA and STANJAN equilibrium codes have been developed by a simple modification of their input files. To evaluate the performance of these subroutines, test calculations have been made for the hydrogen - oxygen (H - O) and carbon - hydrogen - oxygen - nitrogen (C - H - O - N) systems with various combinations of constraints on the elements, the total moles and the free valence (number of unpaired electrons) in the system. The allowed domain of the constraints was determined and both interior and boundary points were investigated for several temperature and pressure conditions. The results showed that STANJAN was superior to NASA both in convergence and speed under all conditions investigated.