A powerful research tool for thermodynamic and kinetic modelling of chemical speciation in complex aqueous environments. It is designed to solve problems requiring expert knowledge of solution chemistry. It currently comprises over 250 programs, 2000 subroutines and 234,000 lines of Fortran code. The software is highly portable and has been implemented under VAX-VMS, Unix, MS-DOS, Windows95/98 and Windows-NT and a number of other operating systems. In most JESS modelling, the speciation is calculated using known thermodynamic parameters for the relevant reactions in aqueous media. All types of chemical equilibria can be modelled including protonation, complex formation, redox, solubility and adsorption interactions. The data are taken from the JESS databases, transformed into a thermodynamically consistent set of equations and solved. Automation of this procedure, achieved by a sequence of JESS programs, is the key to good equilibrium modelling.