Conventional level set-based approaches have an inherent difficulty in tracking miscible fluids due to its discrete treatment for interface. This paper proposes a unified framework to efficiently handle both miscible and immiscible fluid simulations. Based on the chemical potential energy, our method describes the evolution of multiple fluids as time-varying concentration fields. Handling of multiple fluids is straightforward and, unlike level set methods, ad hoc reinitialization or fictitious particle deployment is not necessary. For numerical computation of the Navier-Stokes equations, we adopt advanced lattice Boltzmann methods (LBMs) for computational efficiency. The experiments show that our approach works well with immiscible fluids, miscible fluids, and interaction With objects. Copyright (C) 2008 John Wiley & Sons, Ltd.