The mixing of a single-component or multi-component hydrocarbon (HC) droplet in supercritical or near-critical water (SCW/NCW) is modeled. Transport, thermodynamics, and phase equilibrium sub-models are used to estimate the relevant physical properties. We use a generalized Maxwell-Stefan (MS) expression to model the multi-component mass transfer and a diffusion driving force expressed in terms of fugacity gradients to account for effects of non-ideality on mass fluxes. We compare the ideal and non-ideal diffusive driving forces for different mixing conditions and different HCs, and show that when the mixing temperature is close to or greater than the upper critical solution temperature (UCST), the non-ideal driving force model predicts a much slower mixing process and higher concentrations of the heavier HC than the ideal driving force, due to the presence of a diffusion barrier captured by the non-ideal driving force model. (C) 2015 Elsevier B.V. All rights reserved.