Changes in sodium and potassium content of the leaves of five Acer trees are recorded during two seasons. In general, sodium contents remained constant during leaf maturity and increased during senescence, whereas potassium contents increased during leaf maturity and decreased during senescence. Short-term fluctuations in sodium content during one season could be related to periods of rainfall preceding some of the days of sampling. Experiments in which excised leaves were washed in a spray of de-ionized water showed that the proportion of leaf potassium which could be removed increased as senescence progressed, and the quantity leached was approximately proportional to the area of the leaf which had yellowed.With increasing leaf age, the amount of potassium which diffused quickly out of discs cut from the leaves increased, whereas diffusion of sodium became less. Discs cut from senescent leaves were capable of absorbing sodium and it is suggested that this ability might account for the increase in sodium content of Acer leaves during senescence.