Throughfall chemistry was studied in a mature Sitka spruce plantation in order to investigate canopy interactions, such as nitrogen absorption, cation leaching, and neutralization of rainfall passing through the canopy. The plantation had been exposed to six different simulated mist treatments including N (NH 4NO 3) and S (H 2SO 4 at pH 2.5) in four replicated blocks since 1996. Throughfall and rainfall were collected from May to September 2000. The results showed that 30–35% of the applied N was retained by the canopy. There were linear relationships between the loss of H + and increased K +, Mg 2+ and Ca 2+ deposition through the canopy. However these increases in K +, Mg 2+ and Ca 2+ deposition accounted for only about 50% of total neutralization of the acidity. The relationship between the anion deficits in throughfall and the loss of H + implied that weak organic acid anions were involved in the neutralization of the acidity in throughfall. Weak organic acids may be involved in neutralizing acidity of throughfall.