A study involving methylated metalloid species reveals that high concns. of these compds. can be produced by composting of biomaterials. More than 100 mg/kg of methylated arsenic species, several mg/kg of methylated tin and antimony species, and 100 pg/kg of Me2Te were detected in the compost by hydride generation. Volatile compds. of these elements were detected in the gas phase within the compost heaps, as well as methylated species of selenium, iodine, and bismuth. Organometalloid compds. were formed rapidly within the compost and remained stable over a period of several weeks. In fresh compost, the concn. dropped rapidly and almost disappeared after 12 wk in the garden compost. The results for samples from a composting facility using compost piles for composting were very similar to normal garden composts.