Leaf attributes of four savanna tree species were measured along a rainfall gradient (1650–950 mm per annum) in the Australian monsoon tropics. As the mean annual rainfall decreased, leaf thickness increased for three of these four species. However, a corresponding decrease in leaf density for two species meant that leaf mass per area increased significantly only for one species. Physiological measurements were made during both the wet and dry seasons on comparable stands of vegetation near the extremes and middle of this gradient. Assimilation per unit mass was similar at all three sites but assimilation per leaf area was higher at the drier sites because leaves were thicker with higher mass per area. These results probably reflect reduced tree density and leaf area index at the drier sites, which offsets the lower rainfall, potentially allowing similar rates of assimilation per unit carbohydrate invested in leaves.