Methods involving polyoxymethylene (POM) as a passive sampler are increasing in popularity to assess contaminant freely dissolved porewater concentrations in soils and sediments. These methods require contaminant-specific POM-water partition coefficients, K-POM. Certain methods for determining K-POM perform reproducibly (within 0.2 log units). However, other methods can give highly varying K-POM values (up to 2 log units), especially for polycyclic aromatic hydrocarbons (PAHs). To account for this variation, the authors tested the influence of key methodological components in K-POM determinations, including POM thickness, extraction procedures, and environmental temperature and salinity, as well as uptake kinetics in mixed and static systems. All inconsistencies in the peer-reviewed literature can be accounted for by the likelihood that thick POM materials (500m or thicker) do not achieve equilibrium (causing negative biases up to 1 log unit), or that certain POM extraction procedures do not ensure quantitative extraction (causing negative biases up to 2 log units). Temperature can also influence K-POM, although all previous literature studies were carried out at room temperature. The present study found that K-POM values at room temperature are independent (within 0.2 log units) of POM manufacture method, of thickness between 17m and 80m, and of salinity between 0% and 10%. Regarding kinetics, monochloro- to hexachloro-polychlorinated biphenyls (PCBs) were within 0.2 log units of equilibrium after 28d in the mixed system, but only dichloro-PCBs achieved near equilibrium after 126d in the static system. Based on these insights, recommended methods and K-POM values to facilitate interlaboratory reproducibility are presented.