Two sediment passive samplers, polyethylene (PE) and polyoxymethylene (POM), were compared and mutually validated for measuring freely dissolved concentrations (C-free) of polychlorinated biphenyls (PCBs) in sediment porewater. PE and POM strips in commonly used dimensions (30 and 76 mu m in thickness, respectively) were exposed to sediment slurries for 28 d. The C-free values calculated using literature polymer-water partition coefficients were consistently higher for PE than for POM by a factor of 2 on average. Time series experiments over 96 d show that 28 d are sufficient for attaining partition equilibrium of PCBs for PE, whereas even 96 d may not be enough for POM. To gain additional insight, POM and PE strips were co-exposed to bovine serum albumin suspension spiked with PCBs. The POM/PE concentration ratios increased over 56 d, and the ratios at 28-d were in agreement with the POM-to-PE ratios of PCB concentrations from the 28-d sediment slurry experiments. This agreement suggests that the use of apparent POM-water partition coefficients (i.e., non-equilibrium concentration ratios) suitable for a 28-d exposure to sediment slurries may correct the non-attainment of equilibrium and could provide more accurate Cfree values. (C) 2017 Elsevier Ltd. All rights reserved.