A computationally efficient model to calculate gas/aerosol partitioning of semivolatile inorganic aerosol components has been developed for use in global atmospheric chemistry and climate models. We introduce an approximate method for the activity coefficient calculation that directly relates aerosol activity coefficients to the ambient relative humidity, assuming chemical equilibrium. We demonstrate that this method provides an alternative for the computationally expensive iterative activity coefficient calculation methods presently used in thermodynamic gas/aerosol models. The gain of our method is that the entire system of the gas/aerosol equilibrium partitioning can be solved noniteratively, a substantial advantage in global modeling. We show that our equilibrium simplified aerosol model (EQSAM) yields results similar to those of current state-of-the-art equilibrium models.