The static respiration activity index test was designed to assess the biodegradability (or biostability) of compost samples by monitoring their oxygen consumption rate under cultivation conditions. However, as revealed in this study, the measured activity index correlated not only with the biochemical characteristics of the tested sample but also with the operational conditions, including sample quantity, sample moisture content and cultivation temperature. It was speculated that this observation could be attributed to the limited oxygen transport to the sample. Hence the oxygen transport and consumption in the tested sample were modelled using a one-dimensional, pseudo-steady state model to estimate the relative significance of mass transfer resistance to the bioreaction rate. A so-called critical thickness of the sample, above which mass transfer resistance became significant, was identified and assessed under various experimental conditions. The possible oxygen limitation for the static respiration test to estimate the activity index of compost samples was critically assessed. (c) 2006 Society of Chemical Industry.