A pyritic refractory gold ore was oxidized in short and tall isothermal columns at different temperatures (22, 45 and 65 degreesC) with consortia of mesophiles, moderate and extreme thermophiles under a variety of experimental conditions. These experiments were performed as part of a heap leach model validation program. Severe air channeling and bed compaction occurred in columns as short as 2 m packed with non-agglomerated ore particles. Similar problems were encountered in columns loaded with 9 mm agglomerates prepared with smaller particles (<3.3 mm). Although aeration was key to maintaining high potentials, neither the irrigation flow rate, nor the inoculation procedure, nor CO2 enrichment had any significant effect on the oxidation kinetics at ambient temperature. Elemental sulfur oxidation was more rapid at elevated temperatures. Reasons why sulfide oxidation kinetics in the 65 degreesC column were comparable to the 45 degreesC test, and thus slower than predicted by the intrinsic Arrhenius activation energy of pyrite, are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.