Bioaccumulation of polybrominated diphenyl ethers (PBDEs) in marine and terrestrial organisms is of great environmental concern. Understanding the partitioning of PBDEs to biological phases is crucial for elucidating their bioaccumulation behavior. In this study, partitioning coefficients of PBDEs from water to phospholipid liposomes (K_lipw) and to bovine serum albumin (BSA) (K_BSAw) were measured using a polymer depletion method. K_lipw values measured in two laboratories are in an excellent agreement (within 0.14 log units), indicating a high method repeatability. The values of log K_lipw and log K_BSAw are in the range of 6.3–8.2 and 5.1–7.0, respectively, representing the highest of these partition coefficients measured to date. Log K_lipw values for PBDEs are similar to the log of the octanol–water partition coefficients (K_ow) (log K_lipw − log K_ow = −0.06 ± 0.23; mean ± SD of 7 congeners), consistent with the data of chlorobenzenes and polychlorinated biphenyls (PCBs) reported in the literature. No hydrophobicity cutoff was observed for partitioning to the liposome membrane. Log K_BSAw values for PBDEs increase linearly with log K_ow and do not follow the non-linear trend observed for polycyclic aromatic hydrocarbons (PAHs). This difference between PBDEs and PAHs suggests specific binding mechanisms to serum albumin even for hydrophobic compounds.