Spatial and temporal variation of isoproturon (IPU) residues (IPUr), and their sorption (Kd) and desorption (Kapp) coefficients were monitored at the field scale in the cultivated horizon of a clay soil following pre- and postemergent IPU applications to a winter barley crop. The half life in 25 subplots ranged from 31 to 483 days with a median value of 90 days. Kapp increased as IPUr declined, ranging from a median value of 9.9 L/kg, 27 days after the pre-emergent application (autumn 1991) to 77.8 L/kg four days prior to the post-emergent application (spring of 1992). Kd's were more consistent, with median values ranging from 3.7 to 6.4 L/kg over the same period. However, median Kd's of 7.1 and 8.7 L/kg in May and June 1992 respectively would suggest that increases may occur during the summer months when temperatures increase and the soil dries. Sorption nonideality (Kd/Kapp) was observed on each sampling occasion, ranging from approximately 0.5, 27 days after the pre-emergent herbicide application, to approximately 0.2 four days prior to the post-emergent application. All parameters were positively skewed. Variation of all parameters at the local scale (i.e. within 1 m2) was less than that at the field scale, but greater than that associated with the experimental methods employed indicating that both intrinsic and extrinsic factors influenced the distribution of IPUr, Kd and Kapp. Variability of IPUr and Kd on individual sampling dates was consistent with coefficients of variation generally ranging from 26 to 44% and from 18 to 44% respectively. Variability of Kapp was more erratic, increasing to over 200% and 100% following two major rainstorm events in November 1991 and February 1992 repectively. Normalization of Kd's on a soil organic matter basis failed to reduce the variability observed. The implications of these results for contaminant transport and fate modelling are discussed.