Adjuvants are used to improve pesticides' performance. It is expected that adjuvants should increase sorption and persistence, as well as decrease mobility of pesticides in soils. Impact of the "Grounded" brand adjuvant on the behaviour of two herbicides, pendimethalin and dimethenamid-P, was investigated in a Haplic Chemozern. Both herbicides were tested in a laboratory batch sorption experiment with and without adjuvant. The sorption experiment showed that adjuvant negligibly increased dimethenamid-P sorption (K-F = 2.12 and 2.15 cm(3/)n mu g(1-1/n) g(-1)) but significantly increased pendimethalin sorption (K-F = 270.1 and 3096.4 cm(3/n) mu g(1-1/n) g(-1)). In field conditions, both herbicides were retained mainly in the topsoil layer (0-5 cm). The pendimethalin dissipation half-lives were similar for all treatments (ranging from 43.0 to 44.6 days) and were not influenced by either irrigation (p = 0.86) or adjuvant (p = 0.9). The dimethenamid-P dissipation half-lives ranged from 8.8 days for irrigated treatment without adjuvant to 12.9 days for non -irrigated treatment with adjuvant. Dimethenamid-P dissipation half-life in treatments with adjuvant was significantly longer (p = 0.049) than was half-life in a treatment without adjuvant. Significantly longer dissipation half-life was observed also in non irrigated treatments than in irrigated treatments (p = 0.044).