The isothermal decomposition of hydrogen peroxide on a CuCr catalyst in laboratory scale trickle bed reactor was used to test model predictions of the dependence of liquid reactant conversion on space time for different operating conditions. It is assured that the decomposition of hydrogen peroxide is a first order liquid-limited reaction. Comparison of model predictions and experimental data indicates that both external mass transfer effects and incomplete external catalyst wetting need to be accounted for. Dudukovic’s (1977) approximate model for the catalyst effectiveness factor adequately simulates both effects.