The reactions and corresponding system of equations for the inorganic SO4 2--NO3 --NH4 + system have been studied with a new heterogeneous partitioning code, HETV. The code is based on the algorithms of ISORROPIA (Nenes et al., Aquat. Geochem. 4 (1998) 123; Atmos. Environ. 33 (1999) 1553), but was constructed for maximum computational efficiency on a vector supercomputer using the "vectorization by gridpoint" technique (Jacobson and Turco, Atmos. Environ. 28 (1994) 273). The new code was tested on two common computer architectures (vector supercomputers and UNIX workstations). The new code requires 1/38-1/89 of the processing time of the original ISORROPIA code on the vector supercomputer and produces equivalent results. A rigorous testing procedure was employed to compare the results of the two codes for conditions encompassing the typical range of SO4 2-, NO3 - and NH4 + concentrations found in the ambient atmosphere. The procedure was instrumental in the creation of several improvements to the numerical methods for the solution of the system of equations. Both the improvements and the testing procedure are described in detail, and are recommended for future code development of this nature. © 2003 Elsevier Science Ltd. All rights reserved.