Use of total pressure modulation to improve reactor performance has attracted attention for about three decades from a several different fields: chemical engineering, metallurgical engineering and biochemical engineering. Ultrasonic excitation of both mass transfer and reaction systems has been discussed in the literature extensively. Although ultrasonic pressure waves are low-amplitude, high-frequency, total-pressure oscillations, this review will consider such excitation just peripherally. Our attention will be focussed on forcing at higher amplitudes and much lower frequencies. These are directed at enhancing mass transfer in porous catalyst particles. High-amplitude forcing has been studied for other reactor operating variables such as feed composition and temperature. Modulation in which an abrupt pressure reduction is employed can stimulate reaction systems that are product-inhibited. There is a no literature in this area, but interest is growing because an extension of the principles involved leads to pressure-swing reactors. (C) 2004 Published by Elsevier Ltd.