A two-dimensional finite-element model was developed to predict coupled transient flow and multicomponent transport of organic chemicals which can partition between NAPL, water, gas and solid phases in porous media under the assumption of local chemical equilibrium. Gas-phase pressure gradients are assumed negligible and liquid flow equations are solved simultaneously using an upstream weighted solution method with time lagged interphase mass-transfer terms and phase densities. Phase-summed component transport equations are solved serially after computation of the velocity field also by an upstream weighted finite-element method. Mass-transfer rates are evaluated from individual phase transport equations by back-substitution and corrected for mass-balance errors. Results of hypothetical simulations of the transport of slightly soluble and volatile organics in three-phase porous media indicate that mass-transfer rate and fluid density updating have negligible effects during periods of highly transient NAPL migration but become important for long-term simulations. Due to low solubilities of environmentally important organic liquids, the efficiency of organic removal by aqueous-phase dissolution and transport can be very slow. Gas-phase diffusion can have a significant influence on the mass transport of organics with large Henry's constant. (Copyright (c) 1990 Elsevier Science Publishers B.V.)