Experimental results are presented on the liquid back-mixing of distillation trays. The tests were conducted in the 1.22 in diameter column of the FRI distillation unit using different systems at various pressures. Both bubble-cap and sieve trays with different designs were tested. For each test, a pulse of nonvolatile dye was injected into the reflux stream and the liquid on several trays was continuously monitored to determine the effects of the trays on this pulse. The degree of liquid back-mixing on the tray was then deduced as a function of the change in the shape of the pulse. The results show that liquid back-mixing is strongly dependent on tray designs and system properties. Certain trays and systems had a high degree of plug flow; others were highly mixed. The test results indicate that the liquid back-mixing occurring in the tray deck is much larger than that in the downcomer. Mixing pool numbers and eddy diffusivity were computed from the experimental data. The results are compared to predictions from several literature models. The predictions from the Bennett et al. and Zuiderweg correlations agree reasonably well with the experimental results.