This study was conducted to examine adsorption and the utility of Koc for the pesticides carbofuran (2,3‐dihydro‐2,2‐dimethyl‐7‐benzofuranyl methylcarbamate), metalaxyl (N‐(2,6‐dimethylphenyl)‐N‐(2‐methoxyacetyl) alanine methyl ester), and simazine (6 chloro‐?,?'‐diethyl‐1,3,5‐triazine‐2,4‐diamine). Adsorption parameters for these pesticides were measured in the laboratory on samples from 12 western Washington soil layers. Predicted transport of the 3 pesticides using Koc values from the literature and from this study as model inputs were compared with field observations. Carbofuran was the least strongly adsorbed of the three pesticides. Carbofuran adsorption correlated well with soil organic carbon content. Koc values calculated for carbofuran were similar over a range of soils from both this study and other literature. Metalaxyl appeared to be preferentially adsorbed on soil mineral surfaces, and meaningful Koc estimates could not be obtained. Simazine behavior was intermediate between carbofuran and metalaxyl. For carbofuran and simazine, reasonable predictions of pesticide transport could be made using published Koc data for model inputs. Site‐specific adsorption parameters are probably necessary to characterize metalaxyl transport.