[1] Tropical savanna (locally known as cerrado) composes 24% of Brazil and is characterized by high climatic variation; however, patterns of energy exchange are poorly understood, especially for mixed grasslands (locally known as campo sujo). We used eddy covariance to measure latent (L_e) and sensible (H) heat flux of a mixed grassland and linked meteorological and remote sensing data to determine the controls on these fluxes. We hypothesized that (1) seasonal variations in H and L_e would be large due to variations in precipitation; (2) ecosystem phenology, estimated using the enhanced vegetation index (EVI), would be the best predictor of seasonal variation in L_e; and (3) cerrado, transitional, and humid evergreen forests would have similar rates of average annual L_e despite large seasonal variation in cerrado L_e. Our data suggest that campo sujo exhibits large seasonal fluctuations in energy balance that are driven by rainfall and that responses to rainfall pulses are rapid and dynamic, especially during the dry season. Path analysis indicated that temporal variations in the EVI did not significantly affect L_e or G_c, but this was because all three variables (EVI, L_e, and G_c) responded similarly to temporal variations in surface water availability. Compared to other tropical ecosystems, wetter sites had higher rates of L_e during the dry season but similar rates during the wet season when water was not limiting. Over annual time periods, average rates of L_e increased significantly as average annual rainfall increased, due to dry-season water limitations in the more seasonal tropical ecosystems.