The aim of this paper is to present two methods for the measurement of the residence time distributions of solid particles in reactors with pneumatic transport flow. Both methods are based on the measurement of the "concentration" of solid by light absorption following the Beer Lambert law. The first method involves the capture of particles on an adhesive surface followed by analysis with a time delay; the second method permits direct analysis in the reactor. A comparison between the two methods is presented, with particular emphasis on their respective advantages and disadvantages. Both measurement methods are capable of attaining short residence times, in the order of 0.1 s. (C) 2001 Elsevier Science B.V. All rights reserved.