A process for preparing phthalides of the general formula I, ##STR1## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently hydrogen, C.sub.1-C.sub.4 -alkyl or C.sub.1 -C.sub.4 -alkoxy, by hydrogenating phthalic anhydrides of the general formula II, ##STR2## where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are each as defined above, with hydrogen in the presence of suspended catalysts of the Raney type in a hydrogenation apparatus by using a mixing apparatus to mix a liquid phase, which includes the catalyst and the phthalic anhydride used and resulting phthalide, and a gas phase, which includes the hydrogenating hydrogen, comprises using the mixing apparatus to introduce a mixing intensity of at least 50 W/l into the liquid phase.