The complementary relationship (CR) between potential evaporation (LEp) and actual evaporation (LE) is widely used to explain the evaporation paradox and to estimate LE, in which wet environment evaporation (LEw) is usually calculated using the Priestley–Taylor equation. However, in many studies on the CR, it has been found that the Priestley–Taylor parameter α is not a constant. Through seasonal variation of α for estimating LEw in the CR, this paper analyses its seasonal variability. Based on flux observation data at two flux experiment sites (Kogma in Thailand and Weishan in China) in the Asian monsoon region, seasonal variability of the CR is detected, i.e. the α value is larger in winter than in summer. This seasonal variability might be caused by seasonal variability in the transport of water vapor and sensible heat between oceans and continent. The monsoon increases air humidity and lowers air temperature in summer, which leads to a decrease in α; it increases atmospheric air temperature and vapor content in winter, increasing α. Nevertheless, during May–September, α has a range of 1.06–1.16 at the Kogma site and 1.00–1.36 at the Weishan site, which is approximate to the typical range 1.1–1.4. Copyright © 2012 John Wiley & Sons, Ltd.