In‐lake alkalinity generation (IAG) is important in regulation of alkalinity in lakes with long residence times, particularly seepage lakes. An IAG model based on input/output modeling concepts is presented that describes budgets for each ion involved in alkalinity regulation by a single differential equation that includes inputs, outputs, and a first‐order sink term. These equations are linked to an alkalinity balance equation that includes inputs, outputs, IAG (by sulfate and nitrate reduction), and internal alkalinity consumption (by ammonium assimilation). Calibration using published lake budgets shows that rate constants are generally similar among soft water lakes (kSO4 ≈ 0.5 m/yr; kNO3 ≈ 1.3 yr−1; kNH4 ≈ 1.5 yr−1). Sensitivity analysis shows that predicted alkalinity is sensitive to water/residence time, but less sensitive to modest changes in rate constants. The model reflects the homeostatic nature of internal alkalinity generation, in which internal alkalinity production increases with increasing acid input and decreases with decreasing acid inputs of HNO3 or H2SO4.