Calcite solubility is of major importance to geologists and soil scientists, and yet attempts to characterize the system in terms of basic chemical relationships have met with limited success. The model reported herein adequately describes calcite equilibria in aqueous solutions open to the air that have practical significance to both geologists and soil scientists. Equations accounting for activity coefficients and various ion-pair formations were sufficient to describe calcite equilibrium in solutions of widely varying ionic characteristics. Sixty out of 73 solutions selected from the literature yielded calculated Kisp values for calcite ranging from 2.6 to 4.0 x 10−9with an average of 3.3 x 10−9.