Although vast areas in tropical regions have weathered soils with low potassium ( K ) levels, little is known about the effects of K supply on the photosynthetic physiology of trees. This study assessed the effects of K and sodium ( Na ) supply on the diffusional and biochemical limitations to photosynthesis in Eucalyptus grandis leaves. A field experiment comparing treatments receiving K (+ K ) or Na (+ Na ) with a control treatment ( C ) was set up in a K ‐deficient soil. The net CO₂ assimilation rates were twice as high in + K and 1.6 times higher in + Na than in the C as a result of lower stomatal and mesophyll resistance to CO₂ diffusion and higher photosynthetic capacity. The starch content was higher and soluble sugar was lower in + K than in C and + Na , suggesting that K starvation disturbed carbon storage and transport. The specific leaf area, leaf thickness, parenchyma thickness, stomatal size and intercellular air spaces increased in + K and + Na compared to C . Nitrogen and chlorophyll concentrations were also higher in + K and + Na than in C . These results suggest a strong relationship between the K and Na supply to E. grandis trees and the functional and structural limitations to CO₂ assimilation rates.