AbstractAerated and nonaerated windrow composts of explosives‐contaminated sediments at the Umatilla Army Depot Activity (UMDA, Hermiston, OR) were characterized chemically and toxicologically as a function of composting time. The concentrations of explosives in organic solvent extracts of the composts and in the aqueous leachates of the composts, the bacterial mutagenicity of organic solvent extracts from the composts, and the toxicity of aqueous leachates from the composts toCeriodaphnia dubiaall declined rapidly (97.5%, respectively). Extractable mutagenicity declined 99.7 and 97.9%, respectively, for strains TA‐98 and TA‐100 (without S‐9 metabolic activation) in the nonaerated compost. The two types (aerated and nonaerated) of windrow composts had about the same level of efficiency in lowering leachable toxicity (by 92% and 87%, based on reductions inC. dubiasurvival and fecundity, respectively). Thus, windrow composting appeared to be at least as effective as static‐pile and mechanically stirred composting evaluated previously. Windrow composting also appeared to be somewhat more effective for HMX trans