A fiber optic technique was used to obtain point residence time distributions at several points on an 8 ft. diam. sieve tray in hydrocarbon service. Residence time patterns were obtained over a wide range of vapor and liquid loadings with the isopropyl alcohol–water system and at both ends of the cyclohexane toluene system. The data reveal severe flow nonuniformities on the tray including nonuniform velocity distribution and retrograde flow near the column walls. The reasons for this behavior are discussed.