Abstract A mathematical model is derived for the evaluation of the relative velocity between the gas and the pellets in two configurations of the spinning catalyst basket reactor. This slip velocity is then used to calculate the particle Reynolds number and hence mass transfer coefficients from well known correlations for mass transfer in packed beds. Experimentally determined mass transfer coefficients were found to be in good agreement with the model. The effect on mass transfer rates of the position of the pellet and rotor dimensions for the cruciform spinning catalyst basket reactor is seen to be well predicted from the model. The performance of a typical spinning catalyst basket reactor is calculated when a catalytic oxidation reaction takes place at various jacket temperatures and rotational speeds.