Abstract Volumetric mass transfer coefficients ( k La ), and gas holdups (ε G ) were studied for the oxygen transfer into organic slurries. Aluminium oxide, polyvinylchloride and polyethylene were used as solid particles in ligroin and tetralin. It was found that the presence of particles can affect the mass transfer remarkably. Solids with high densities can increase k La in small concentrations for liquids with low density and viscosity. When the density difference is small or the liquid viscosity is high, the slurry behaves as a pseudohomogeneous phase. Successful correlations were proposed for k La and ε G as a function of the effective suspension viscosity.