The effect of fluid density and viscosity on dispersion in miscible displacements in porous media is examined. Miscible displacement experiments with fluid pairs having density and viscosity differences are carried out in a Plexiglas column containing a homogeneous and isotropic sand pack. Tracer tests and tests of both stable and unstable miscible displacement are conducted using NaCl and glycerine solutions. Concentration breakthrough curves are measured through an electrical monitoring technique. Following the conventional convection -dispersion formulation, the dispersion coefficient is determined by performing a least squares fit to the measured concentration breakthrough curves. It is found that for stable displacements dispersion coefficients drop continually when density differences increase or when viscosity ratios of the displaced and displacing fluid decrease. In the unstable case the dispersion coefficients increase with both density and viscosity differences.