A computer program, CHEMEQUIL-2 (CHEMical EQUILibrium), based on interfacing an iterative algorithm with the Newton-Raphson method, for calculating equilibrium compositions in aqueous mixtures of metals and ligands, is described. The program is also capable of simulating acid-base titrations. It has been compared with MINIQUAD, COMPLEX and MINEQL with respect to execution time and memory requirements. As a result of algorithm development and program design, CHEMEQUIL-2 offers considerable savings in both execution time (by 1-2 orders of magnitude) and memory requirements, especially for large problems, compared to these programs. The computational efficiency of CHEMEQUIL-2 makes it well suited for use in hydrogeochemieal transport models.