Triphase catalysis (TPC) is a practically attractive way of carrying out phase transfer catalyzed reactions. The analysis of triphase catalytic systems requires a knowledge of mass transfer coefficients, but no correlations for estimating these are available. Therefore, a new rotating disk contactor (RDC) for measuring the external mass transfer coefficients of TPC systems has been designed and operated. Details of the design and construction of the contactor are discussed. In order to test the apparatus, experiments were carried out on a test system : synthesis of benzyl acetate from benzyl chloride and sodium acetate with polymer supported tributylmethylammonium chloride as the immobilized phase transfer catalyst. This reaction was found to be mass transfer controlled at low agitation speeds, and it was therefore possible to determine the external mass transfer coefficient as a function of the bulk agitation speed. The values ranged from 1.4 x 10(-3) to 2.0 x 10(-3) cm/min.