Simazine (6-chloro-N2,N4-diethyl-1,3,5-triazine-2,4-diamine) is the second most commonly detected pesticide in surface and ground waters in the U.S., Europe, and Australia, presumeably due to a combination of simazine's solubility, low sorption, and relatively high persistence in both soil and water. The number of detects have raised concerns regarding its impact on human health and on all aspects of aquatic and terrestrial ecosystems. The objective of the present study was to develop a solvent extraction procedure, which would allow prediction of simazine bioavailability in aged soils. Bioavailability was characterized by measuring the amounts of simazine mineralized by Pseudomonas sp. strain ADP, a simazine-degrading bacterium, in aged soils. These data were then correlated to the amounts of simazine extracted by a sequential solvent extraction procedure. ¹⁴C-simazine was applied to two glacial soils from the U.S. (US-1 and US-2) and to four tropical soils from Brazil (BR-1 and BR-2) and Hawaii (HW-1 and HW-2) at rate of 2 mg kg^-1. The soils were moistened to -33 kPa and then incubated for up to 55 d. At 0, 7, 14, 28, and 55 d after simazine application, the soils were inoculated with Pseudomonas sp. strain ADP and the ¹⁴CO₂ evolved was monitored for 138 h. The soils were then extracted once with 50 mL of 0.01 N CaCl₂ and then twice with 50 mL of aqueous methanol. The extracted soils were dried, macerated, and combusted to determine nonextractable (bound) ¹⁴C-simazine residues. The ¹⁴C mineralized decreased with aging (from 85 to 14% in the BR-1, from 82 to 20% in the BR-2, from 80 to 39% in the US-1, from 76 to 30% in the US-2, from 75 to 2% in the HW-1, and from 77 to 33% in the HW-2. Simazine bioavailability to Pseudomonas sp. strain ADP was closely correlated to the total amounts of ¹⁴C-simazine sequentially extracted by aqueous and methanol solutions. These microorganisms were capable of mineralizing all the readily-available (water-extractable) and a great portion of the less-readily-available (methanol-extractable) ¹⁴C-simazine. The ¹⁴C-bound residue fraction after bacteria inoculation was the same as before inoculation in all soils. This nonextractable fraction, if it contains ¹⁴C-simazine, may be considered very recalcitrant and consequently unavailable in soils, even to highly efficient microbes. The establishment of such a solvent extraction-bioavailability correlation is very useful since it allows the prediction of simazine bioavailability and helps regulatory decisions by expressing better exposure to humans, animals, or plants, which is often overestimated when using more harsh procedures.