Silicone (polydimethylsiloxane, or PDMS) fluids are widely used in industrial and consumer products and may find their way into municipal sludges that are eventually applied to the land. Earlier studies showed that PDMS degrades in soil to low-molecular-weight, water-soluble silanols of the formula HO–[Si(CH3)2O]n–H. In this study, 200-cs [14C]PDMS (1, 10, 100 mg kg−1) was placed in 50 g Londo sandy clay loam, and the soil was dried to generate the desired silanols in situ. The soil was remoistened and incubated at 25°C, 12% moisture, and 21% O2. The ultimate PDMS degradation product was identified by GC-MS as dimethylsilanediol. During the next 4 months, from 25 to 50% of this substance was either volatilized from the soil, incorporated into the humus, or to a lesser extent oxidized to 14CO2. The latter two processes increased with addition of alfalfa to the soil. No trend was seen in the loss of dimethylsilanediol over different concentrations.