Double-diffusive convection may be an important transport phenomenon in subsurface porous media and fractures. The classic linear stability analysis derived for a porous medium with two components stratified such that each affects the vertical density gradient in an opposing manner predicts double-diffusive finger instability to occur when Rs1+Rs2≥Rsc, where Rs1 and Rs2 are the Rayleigh numbers of the faster and slower diffusing components, respectively, and Rsc is a critical value dependent upon the boundary conditions (0≤Rsc≤4&pgr;2). For cases where Rsc/|Rs1|≪1, the above result can be simplified to -R&rgr;<1/&tgr;, where R&rgr; is the buoyancy ratio of the fluid and &tgr; is the ratio of diffusivities (0<&tgr;<1). We experimentally tested the applicability of both stability criteria for situations where a narrow transition zone exists bounded above and below by constant concentrations and within a domain of uniform permeability. Experiments were conducted in a Hele-Shaw cell using a digital imaging technique which provided pixel-scale (~0.2 mm) resolution of the evolving concentration field during convection. Within experimental error, our experiments support both criteria within their predicted ranges of applicability.¿ 1997 American Geophysical Union