Our previous experiments had shown that PDMS behaves as an inert material during composting, in that even at extremely high concentration it does not harm the composting process, and it does not break down during composting. The present experiment investigates the addition of compost containing PDMS to soil, and the subsequent breakdown of the PDMS in the soil. Compost (approx. 800 mg kg(sup 1) PDMS on a wet weight basis) had been generated in pilot scale reactors from a PDMS-amended sludge which was added to fresh yard waste (leaves, wood chips, and alfalfa) and allowed to mature under composting conditions, as described in our previous report. In the present study, this compost was added to Londo sandy loam soil at a ratio of 1:4 compost:soil (dry weight basis). Samples were placed in plastic flower pots which were buried in the ground to a depth (approx. 45 cm) which set the level of soil in the pots equivalent to the level of soil in the field. Soils were allowed to dry and remoisten in the open air at ambient weather conditions during the summer of 1998. Every two weeks, three flower pots were taken back to the laboratory and soils were extracted with THF (Tetrahydrofuran) and analyzed by GPC-ICP (Gel Permeation Chromatography-Inductively Coupled Plasma). Results showed PDMS degradation in all samples, with the overall PDMS concentration decreasing over time from 0 to 12 weeks. Between 12 and 18 weeks, the concentration of PDMS remained stable. Sampling of soils under the pots at 18 weeks showed that no PDMS or DMSD had leached out of the pots. Analysis of the last samples at 19 weeks indicated that the PDMS concentration had become stratified with depth. The most extensive PDMS degradation (approx. 90% loss) had occurred in the top cm of soil where the soil is most likely to dry out. Slower degradation (approx. 35% loss) had occurred in the 3-4 and 4-5 cm layers, where soil drying should be less frequent. These results suggest that, even though the laboratory half-life of PDMS under ideal conditions is 1-4 weeks, the actual half-life in the field will vary extensively with soil conditions.