A new three-phase mechanically agitated batch laboratory reactor is presented, featuring a cupand-cap holder for powdered catalyst. The apparatus allows precise determination of minute catalyst loadings, accurate control, and stability of process operating conditions, in situ preactivation of the catalyst at any pressure and temperature without external devices for injection or introduction of solids, and zero induction time determinations of reaction rates. The catalytic hydrogenation of vegetable oil derivatives and the evaluation of mass-transfer coefficients are exemplified.