Gas film mass transfer coefficients (k(G)a(t), k(G)a(w)) and liquid film mass transfer coefficients (k(L)a(w)) for packing materials used in biofilters and biotrickling filters for air pollution control were determined experimentally. Lava rock, polyurethane foam cube (PUF), Pall ring, porous ceramic beads, porous ceramic Raschig rings and compost-woodchips mixtures were investigated. The experiments were performed at gas velocities ranging from 100 to 8000 m h(-1) and liquid velocities of 0.1-12 m h(-1), i.e., a wide range that covers most biofilters and biotrickling filters. kGat in biofilter packings ranged from about 500 to 2500 h-1, while k(G)a(w) and kLaw in biotrickling filters ranged from 100 to 8000 h(-1), and 1 to 3000, respectively, depending on the packings and the conditions. This is markedly lower than mass transfer coefficients usually observed for conventional wet scrubbing. The gas film mass transfer coefficient (k(G)a(t)) of 50:50% vol compost-woodchips mixture, a common biofilter packing, was greater than this of a 20% vol compost and 80% woodchips mixture, though the mass transfer was not increased by increasing further the volume fraction of compost. All compost mixtures exhibited a greater gas film mass transfer coefficient than lava rock or other synthetic materials. The mass transfer coefficients of compost mixtures was also influenced by packing method and it was directly proportional to the surface area of the bulking agents added. The gas film mass transfer coefficient (k(G)a(w)) of five biotrickling filter packing materials increased linearly with gas velocity. The effect of liquid on the gas film mass transfer coefficient was not significant. Of all the biotrickling filter packings, the porous ceramic beads had the highest gas and liquid film mass transfer coefficients followed by lava rock, porous ceramic rings, I in Pall ring and PUF cubes. The liquid film mass transfer coefficient (kLaw) was directly proportional to liquid velocity and the effect of gas velocity was negligible. Several correlations allowing prediction of mass transfer coefficients are presented in Part 2 of this paper. (c) 2007 Elsevier Ltd. All rights reserved.