Numerous reports show that both transfer and uptake of poorly-water soluble substrates are significantly enhanced in two-phase partitioning bioreactors (TPPBs). A number of hypotheses have been put forward to explain these enhancements and among them, the occurrence of direct substrate or oxygen uptake from the vector/water interface has been suggested. The objective of this paper was to quantify the direct oxygen uptake from the vector/water interface in a culture of Pseudomonas putida, performed in a stirred tank reactor, using glucose as substrate and silicone oil as vector. Despite of a sufficient dissolved O-2 concentration in the vector phase (17 mg l(-1)) and a significant vector surface area (4,000 m(-1)) no significant direct O-2 uptake from the vector/water interface was observed, compared to O-2 uptake from the aqueous phase. From these results it was concluded that, direct O-2 or substrate uptake from the vector/water interface might not be significant in TPPBs.