We used a continuous 9 year (2003-2011) eddy flux time series with 30min resolution to examine water use efficiency in a tropical rainforest and determine its environmental controls. The multiyear mean water use efficiency (W-ue) of this rainforest was 3.160.33 gC per kg H2O, which is close to that of boreal forests, but higher than subtropical forests, and lower than temperate forests. The water vapor deficit (V-PD) had a strong impact on instantaneous W-ue, in the manner predicted by stomatal optimization theory. At the seasonal scale, temperature was the dominant controller of W-ue. The negative correlation between temperature and W-ue was probably caused by high continuous photosynthesis during low-temperature periods. The V-PD did not correlate with W-ue at the interannual scale. No interannual trend was detected in W-ue or inherent water use efficiency (W-ei), either annually or seasonally. The fact that no increasing trend of W-ei was found in the studied tropical rainforest, along with other evidence of CO2 stimulation in tropical rainforests, requires special attention and data validation. There was no significant difference between W-ue during a drought and the 9 year mean values in the forest we studied, but we found that dry season transpiration (T-r) was consistently lower during the drought compared to the mean values. Finally, whether W-ue increases or decreases during a drought is determined by the drought sensitivity of gross primary production (G(PP)).