The allelopathic potential of isothiocyanates (ITC) release by turnip-rape mulch [Brassica rapa (Rapifera Groupe)-Brassica napus L.] Six different ITCs were identified from chopped turnip-rape by HPLC-DAD/HPLC-MS. All plant parts contained 2-phenylethyl-ITC. In the shoot n-butyl and 3-butyl-ITC dominated. Younger leaves, flowers, and buds also contained small ampunts of benzyl and allyl-ITC. Futhermore, marginal amounts of 4-pentenyl-ITC were detected. In the soil, where turnip-rape mulch was incorporated, only low amounts of ITCs were detected. It was shown that the DT50 of ITCs in soil are verry short. Germination tests with weed seeds in a aqueous ITCs solutions shoed, that aryl-ITCs were the most suppressive compounds. Within the alkyl-ITCs, the activity decreased with increasing molecular mass. The susceptibility of different weed species to ITCs mainly depended on seed size. Smaller seeds tended to be more sensitive. Futher studies demonstrated a high biological activity of ITCs in the vapor phase. n-Butyl-ITC was more suppressive in the vapor phase than in aqueous solution, while 2-phenylethyl-ITC showed the opposite effect. Results demonstrated that weed suppression observed in the field was probably due to the high amounts of ITCs identified in turnip-rape mulch. Isothiocyanates were strong suppressants of germination on tested species-spiny sowthistle [Sonchus asper (L.) Hill], scentless mayweed (Matricaria inodora L.), smooth pigweed (Amaranthus hybridus L.), branyardgrass [Echi-nochloa crusgalli (L.) Beauv.], blackgrass (Alopecurus myosuroides Huds.), and wheat (Triticum aestivum L.)and probably interact with weed seeds in the soil solution and as vapor in soil pores.