Abstract: Oxygen transfer in a flat pJate membrane oxygenator is being studied. A mathematical model for the Cassonian fluid (or Newtonian fluid)convec-tive mass transfer with reversible instantaneous oxygenation is presented. Finite-difference technigue is used to solve the nonlinear boundary-value problem concerned. Calculations indicate that not much difference can be realized between Cassonian and Newtonian fluids. For Newtonian fluids, calculated values of physical absorption conform very well with data given in literature. Experimental studies of the oxygenation of pig blood was undertaken, and results obtained were checked with calculated values, both by treating the hog blood as a Cassonian fluid and as a Newtonian fluid. Close correlations were obtained between the experimental achievement and the calculated values. It was, therefore, concluded that pig blood could be safely treated as a Newtonian fluid.