SUMMARY1 Decomposition of the leaves ofNymphaea albaL. andBetula pubescensEhrh. was studied in two ponds of contrasting pH, buffering capacity and trophic level. Rates of carbon loss, concentrations of nitrogen, protein and phenolics and colonization of the leaf material by macroinvertebrates were studied.2 Decomposition ofB. pubescensleaves was slower than that of N.albain both ponds. Protein and phenolic concentrations were higher in the decomposing N.albamaterial, whereas no significant difference was found for nitrogen concentrations.3 Decomposition of both species was slower in the acid water. The effect of acid water on decomposition rate was similar for both species. Both nitrogen and protein concentrations of N.albaremained higher under acid conditions, whereas no effect of study site was observed on the protein and nitrogen concentrations inB. pubescenslitter. The concentration of phenolic compounds was higher inN. albalitter from the acid pond; no such difference was observed forB. pubescenslitter. The high concentrations of phenolic compounds in N.albalitter from the acid pond probably induced chemical immobilization (tanning reaction), yielding high concentrations of nitrogen and protein in the litter.4 Diversity of the detritivorous macroinvertebrate fauna on the litter of both species was extremely low in the acid pond, while a relatively high diversity occurred in the alkaline water. Exclusion of large macrofauna inhibited decomposition in the alkaline water, while there was no comparable influence in the acid water.5 The effects of low pH on the decomposition of both terrestrial and aquatic macrophyte litter appeared to be caused by interactive effects of low pH on the microbial decomposition, on the detritivorous macroinvertebrates and, in the case ofN. alba, on the resource quality of the