Estrone (E1) and its sulfate conjugate estrone-3-sulfate (E1-3S) are released to the environment in animal wastes in significant amounts, and direct exposure occurs in grazed pasture systems. Both compounds have been shown to potentially contribute to endocrine disruption in wildlife, and knowledge about the sorption behavior of these compounds is necessary for a sound risk assessment. For labile compounds such as E1 and E1-3S, however, the standard protocols might overestimate sorption by not considering metabolite formation or allowing for equilibration that exceeds the commonly reported half-lives of these compounds. We therefore conducted modified batch sorption experiments with 0.005 M calcium chloride (CaCl2) and artificial urine solution to determine the influence of the mediator solution on the sorption of E1 and E1-3S in three pasture soils from New Zealand. Sorption isotherms of both compounds were nonlinear, and the Freundlich equation was found adequate to describe the isotherms. The sorption potential of E1-3S was about one order of magnitude lower than for the free counterpart, and the Kf values significantly changed between the two mediator solutions. The calculation of concentration-dependent effective distribution coefficients (Kdeff) revealed that for a range of realistic exposure concentrations in a grazed dairy system, the common approach of using CaCl2 would deliver incorrect inferences for a sound risk assessment.