Abstract: A mathematical model for describing the start-up period of a plate distillation column operating under total reflux was derived on the basis of the differential-difference equation expressing the material balance on the trays with holdups in the reboiler, trays and condenser, and an analytical solution was presented. The existence of a constant-composition point during the start-up period was proved mathematically. Experimental work on start-up was carried out in a seven-sieve-tray distillation column of 0.17 meter in diameter and operating with a water-acetic acid system. Good agreement was found between experimental data and calculated results. Experiments also showed the evidence of a constant-composition point in the column during start-up.