The mass transfer of phenol and butyl acetate to/from water was studied in two-phase partitioning bioreactors using immiscible organic solvents and solid polymer beads as the partitioning phases in a 5-L stirred tank bioreactor. Virtually instantaneous mass transfer was observed with phenol in water/2-undecanone, and with butyl acetate in water/silicone oil systems. The mass transfer of butyl acetate to silicone oil was rapid irrespective of the viscosity of the partitioning phase. When Hytrel® polymer beads were employed as the partitioning phase, substrate transport to the polymer was found not to be externally mass transfer limited, but rather internally by substrate diffusion into the polymer. In contrast to gaseous, poorly soluble substrates studied in other works, mass transfer of soluble substrates such as phenol and butyl acetate to the polymer was unaffected by impeller speed but rather by polymer mass fraction.