The effect of different factors on the extraction yield in microporous membrane liquid-liquid extraction (MMLLE) was studied in this work. The factors were the extraction temperature, mixing of the donor and acceptor solvents, state of the acceptor (stagnant or flowing), solvent set-up in the membrane unit (organic phase on top or bottom), adsorption of the analytes into the membrane material, and the membrane pore size. The effect of temperature, as it was mediated through the other factors, was of particular interest. Increase in the extraction temperature improved the extraction yields substantially: for naphthalene the extraction yield increased from 14.3% to 17.7% and for benz[a]anthracene from 0.4% to 11.1% when the temperature was raised from 20°C to 80°C. The diffusion coefficients, which were determined with a chromatographic band broadening method, were also clearly increased with temperature: the average diffusion coefficient obtained for naphthalene in water at ambient temperature was 7.92×10–10 m2/s, while at 70°C it was 2.19×10–9 m2/s. The most vigorous mixing of the solvents was obtained with counter-currently flowing acceptor phase and water on top in the membrane block. The adsorption to the membrane material compared with theoretical 100% extraction yield ranged from 0.5 to 5.4% at 80°C when no acceptor phase was present.