The freely dissolved concentration of persistent organic pollutants (POPs) is one of the most important parameters for risk assessment in aquatic environments, due to its proportionality to the chemical activity. Chemical activity difference represents the driving force for a spontaneous contaminant transport, such as water-aquatic biota or water-sediment. Freely dissolved concentrations in sediment pore water can be estimated from the concentrations in a partition-based passive sampler equilibrated in suspensions of contaminated sediment. Equilibration in the sediment/passive sampler system is slow, since concentrations of most POPs in the water phase, which is the main route for mass transfer, are very low. Adding methanol to sediment in suspension increases the POPs' solubility and, consequently, the permeability in the water phase. The resulting higher aqueous concentrations enhance POPs mass transfer up to three times for investigated POPs (polycyclic aromatic hydrocarbons, polychlorinated biphenyls, organochlorine pesticides) and shorten equilibrium attainment to less than 6 weeks. The addition of methanol to the aqueous phase up to a molar fraction of 0.2 changed the POPs equilibrium distribution ratio between sediment and passive sampler by less than a factor of two. As a result, the pore water concentrations of POPs, calculated from their amounts accumulated in a passive sampler, are affected by methanol addition not more than by the same factor.