Pilot-scale reactors have been constructed to mimic the central core of an aerated static bed or in-vessel composting process. The 770 litre reactors were instrumented to measure temporal and spatial variations in temperature, oxygen and moisture content. Experiments were performed with a synthetic food waste (SFW) and digested biosolids using four different aeration rates and two initial moisture contents. An analysis of the temporal and spatial temperature and oxygen profiles has shown the systems replicate well and represent a process with one-dimensional spatial variation. An analysis of oxygen gradients has shown that cumulative oxygen depletion and oxygen depletion rates within the bed increased with increasing aeration rate in the SFW experiments, however, they decreased with increasing aeration rate in the biosolids experiments. The SFW studies showed that a 10% variation in initial moisture content had little influence on cumulative O-2 consumed, but had a significant influence on the location of maximum biological activity within the bed. Maximum temperatures varied from 58 to 74 degrees C in the SFW experiments and from 43 to 60 degrees C in the biosolids experiments. In all experiments the maximum temperatures and the positions where they occurred varied with initial moisture content and aeration rate. In the SFW experiments maximum axial temperature differences coincided with significant axial differences in moisture content, while in the biosolids experiments maximum axial temperature differences coincided with minimal axial differences in moisture content.