A solid-phase fugacity meter was used to measure the soil–air partition coefficients of three semivolatile pesticides (chlorpyrifos, pyrimethanil, and trifluralin) in the absence of additional adjuvants (Ksoil–air,AI), as part of commercial formulations (Ksoil–air,formulation), and as formulation mixtures with an additional spray adjuvant added (Ksoil–air,formulation+spray adjuvant). Chlorpyrifos Ksoil–air,formulation values were also measured over 15–30 °C, allowing for the change in internal energy of the phase transfer reaction (Δsoil–airU) to be calculated and compared to the Δsoil–airU for Ksoil–air,AI from the literature. Measured Ksoil–air values were then used as input parameters in a pesticide volatilization model to understand how their variability affects pesticide volatilization rates under different conditions. Initial experiments conducted at ∼24 °C indicated that all pesticides volatilized more readily in the presence of adjuvants than in their absence and that the additional spray adjuvant had minimal impact. The Δsoil–airU values were 328 and 90 kJ/mol for chlorpyrifos in the absence and presence of formulation adjuvants, respectively, suggesting that adjuvants may weaken or disrupt intermolecular attractions between pesticide molecules and soil. At temperatures below 24.5 °C, modeled chlorpyrifos volatilization rates were higher in the presence of adjuvants than in their absence; however, the opposite occurred at temperatures above 24.5 °C.