A recirculating Gillespie-type still was used to determine the limiting separation factor (K-infinity) for 2-phenylethanol (x) or 2-phenylethanal (x) + water (1 - x). Since the solutions are highly dilute (x < 10(-5)), infinite dilution is assumed. The condensed vapor and liquid samples were analyzed using UV spectroscopy. The confidence intervals are very small, and thus the method has a good reproducibility. The calculated limiting activity coefficient depends highly on the reliability of the pure components vapor pressure data.