In order to protect ground and surface waters from pesticide contamination, broad knowledge is required of sorption-desorption and biodegradation processes since they control the amount of chemical available for pest control and transport. In this study, effects of aging on sorption-desorption and dissipation of simazine (6-chloro-N2,N4-diethyl-1,3,5-triazine-2,4-diamine) were evaluated in glacial (two Midwestern US) and tropical (two Brazilian and two Hawaiian) soils. ¹⁴C-simazine was applied to the six soils at rate of 2 mg kg^-1. Soils were moistened to 75% of field capacity (-33 kPa) and then incubated for 102 days. At 0, 7, 13, 27, 54, and 102 days after application, replicate soil samples were extracted once with 50 mL of 0.01 N CaCl₂ and then twice with 50 mL of aqueous methanol (80:20 v/v methanol/water). The extracted soils were dried, macerated, and combusted to determine nonextractable (bound) 14C-simazine residues. Low to moderate sorption (K_d = 0.7 to 4.0 l kg^-1) and relative slow dissipation rate (t_1/2 = 30 to 75 days) for simazine in five of these soils emphasize their greater potential for mobility in contrast to one Hawaiian soil (K_d = 6.2 l kg^-1 and t_1/2 = 4 days). As expected, the extractable fractions of the simazine residues decreased, whereas the nonextractable fractions increased with soil-pesticide contact time (aging), which resulted in sorption enhancement (higher K_d,app values). The increase in simazine sorption with aging was very likely due to its faster degradation rate in solution and on labile sites in relation to its rate of desorption from soil and its simultaneous diffusion into less accessible sorption sites or “stronger” binding sites. All these findings suggested that K_d values calculated for freshly added simazine may overestimate its real leaching potential and its availability to plants, humans, and all other aquatic and terrestrial living animals.