As part of contributions to code development for the simulation of a severe accident in a pressurised water reactor (PWR), an experimental programme is underway at the "Institut de Radioprotection et de Surete Nucleaire", concerning the mass transfer kinetics of fission products. This programme takes place in the Sisyphe facility which represents a reduced scale (1/5000) pressurised water reactor containment. This programme is to specify the mass transfer coefficients between the sump water and the containment atmosphere using dynamic absorption methods. Measurements are carried out on a simulating volatile fission product, namely oxygen. In the studied operating conditions (isothermal containment at 90 degreesC), there is no difficulty about the determination of the partial mass transfer coefficient in the liquid-phase, and in the case where the aqueous and gaseous phases are macroscopically motionless, this value is of k(L) = 3.0 x 10(-5) m/s. The method chosen for the determination of the gas-phase transfer coefficient (the reactive absorption of oxygen by an aqueous solution of sodium sulphite, with or without catalyst), has shown that the apparent transfer coefficient depends on both the sodium sulphite and catalyst concentrations. This partial mass transfer coefficient tends to a limit value for sufficient sodium sulphite and catalyst concentrations, which is the probable value of the gas-phase transfer coefficient (k(G)), estimated at 10(-4) m/s at 90 degreesC. (C) 2004 Elsevier B.V. All rights reserved.