Gas-solids mass transfer was studied for counter-current flow of gas and millimetre-sized solid particles over an inert packing at dilute phase or trickle flow conditions. Experimental data were obtained from the adsorption of water vapour on 640 and 2200 ¿m diameter molecular sieve spheres at ambient conditions in a test column with a cross-sectional area of 0.06 x 0.06 m2 and a packing height of 0.27 or 0.53 m. The packing consisted of a bank of regularly stacked, 0.01 m diameter bars made of stainless steel. Assuming the effective area for mass transfer to be equal to the external surface area of the spheres, the experimental values of the average gas¿solids mass transfer coefficient were determined to amount to approximately 40¿80% of the values calculated from the well-known Ranz¿Marshall correlation for a single sphere in an undisturbed gas flow. These experimental values were identified as conservative estimates of the actual average gas¿solids mass transfer coefficient because the pore diffusion resistance could not be eliminated completely. A comparison between experimental data on hydrodynamics (gas¿solids momentum transfer) and gas¿solids mass transfer indicated, furthermore, that the influence of particle shielding with interfering concentration boundary layers of different particles was small for the millimetre-sized particles and the experimental conditions investigated. Gas¿solids mass transfer as well as gas¿solids momentum transfer were mainly determined by single-particle flow behaviour.