Differences in concentrations of major ions and silica between interstitial and lake water suggest that some fraction of the total chemical budget of a lake may be supported by diffusional flux of dissolved constituents from sediment into lake water. Estimates of flux from the lake floor give a mean rate of dissolution of sediment or the rate of subaqueous weathering. In the case of two lakes on the Precambrian shield near Kenora, Ontario, flux from the lake floor contributes 15–30%, or less, of the four major cations (Ca, Mg, Na, K) in the lakes. Flux of this magnitude corresponds to dissolution of 0.02–0.04 cm yr‒1 of bulk (solids + water) sediment. The rate of dissolution is less than the typical rates of deposition in postglacial lakes. For such events as lasting changes in the concentrations of dissolved constituents in inflow, new steady‐state concentrations in lake water are attained faster in the presence of flux from the sediment. Mixing of a thin layer of lake floor sediment with lake water may produce transient increases in concentration in lake water comparable in magnitude to the contribution of flux from sediments.