Multistage contactors are often needed to achieve the desired degree of mass transfer. One such contactor is spray column, which can be conveniently used to isolate and purify proteins using aqueous two-phase extraction. The effects of orifice size and addition of NaCl on the overall mass transfer coefficient (K(L)a) of horseradish peroxidase (HRP) and fractional dispersed phase hold up (epsilon(D)) have been studied in a simple spray column using a polyethylene glycol/potassium phosphate aqueous two-phase system. K(L)a and epsilon(D) increased with an increase in dispersed phase superficial velocity (upsilon(s)) as well as capillary diameter. The presence of NaCl increased K(L)a. The overall mass transfer coefficient for HRP normalized for dispersed phase hold up, was found to be independent of upsilon(s) in the range studied. (C) 2002 Elsevier Science Ltd. All rights reserved.