Marginal water-use efficiency plays a critical role in plant carbon-water coupling relationships. We investigated the ecosystem marginal water-use efficiency (lambda) of a tropical seasonal evergreen forest to (1) determine the general pattern of lambda across time, (2) compare different models for calculating lambda, and (3) address how lambda varies with soil water content during different seasons. There was a U-shaped diurnal pattern in lambda, which was higher in the early morning and late afternoon. At other times of the day, lambda was lower and remained constant. Ecosystem lambda was higher in the wet season than in the dry season. All three models successfully captured the diurnal and seasonal patterns of lambda but differed in the calculated absolute values. The idea that lambda is constant on a subdaily scale was partly supported by our study, while a constant lambda was only true when data from the early morning and late afternoon were not included. The lambda increases with soil water content on a seasonal scale, possibly because early morning lambda remained low in dry conditions when the soil water content was low.