We present measurements of water vapor taken using a fast-response Lyman-alpha hygrometer mounted on the NASA ER-2 for the Central Equatorial Pacific Experiment (CEPEX) during March and April 1993. The flights originated from Fiji (17 ¿S, 178 ¿E) and covered a range of potential temperatures up to 470 K. Concurrent ozone measurements and a 1-D model are used to convert potential temperature to effective age of the air in the stratosphere. This analysis provides insight into measured water vapor profiles in the lower tropical stratosphere taken during different seasons on previous aircraft campaigns. It yields a picture of a seasonal cycle consistent with the variation of the average tropical tropopause temperature and an annually averaged mixing ratio of water in air entering the tropical stratosphere of about 4.45 ppmv. ¿ American Geophysical Union 1995