The objective of the experimental investigation was to quantify the axial dispersion of a dilute secondary gas that was subjected to orthogonal thermal and solutal buoyancy gradients in a horizontal annular-like enclosure. Test results showed that a linear axial concentration profile existed at steady-state for the secondary gas. Unheated tests with only solutal buoyancy gradients resulted in a dispersion multiple, D-M, of approximately 8 times molecular diffusion. The combined thermal and solutal buoyancy tests, in which a thermal plume from the simulated heater packages aided in mixing, yielded an axial dispersion multiple coefficient of approximately 23 times molecular diffusion, a 188% increase as compared to solutal buoyancy alone. A correlation describing the experimental data trend for the dispersion multiple as a function of buoyancy ratio, N = Ra-S/Ra-T, was computed to be D-M = 19.2N(-0.061), which is valid for for binary mixtures where the solutal and thermal gradients are orthogonal with Pr and Le of order 1, 0 < Ra-T < 3.5 x 10(6), and 0.1 < N < 0.6. (c) 2013 Elsevier Ltd. All rights reserved.