The concentration of 18O in atmospheric CO2 and H2O is a potentially powerful tracer of ecosystem carbon and water fluxes. In this paper we describe the development of an isotope model (ISOLSM) that simulates the 18O content of canopy water vapor, leaf water, and vertically resolved soil water; leaf photosynthetic 18OC16O (hereinafter C18OO) fluxes; CO2 oxygen isotope exchanges with soil and leaf water; soil CO2 and C18OO diffusive fluxes (including abiotic soil exchange); and ecosystem exchange of H218O and C18OO with the atmosphere. The isotope model is integrated into the land surface model LSM, but coupling with other models should be straightforward. We describe ISOLSM and apply it to evaluate (1) simplified methods of predicting the C18OO soil-surface flux; (2) the impacts on the C18OO soil-surface flux of the soil-gas diffusion coefficient formulation, soil CO2 source distribution, and rooting distribution; (3) the impacts on the C18OO fluxes of carbonic anhydrase (CA) activity in soil and leaves; and (4) the sensitivity of model predictions to the δ18O value of atmospheric water vapor and CO2. Previously published simplified models are unable to capture the seasonal and diurnal variations in the C18OO soil-surface fluxes simulated by ISOLSM. Differences in the assumed soil CO2 production and rooting depth profiles, carbonic anhydrase activity in soil and leaves, and the δ18O value of atmospheric water vapor have substantial impacts on the ecosystem CO2 flux isotopic composition. We conclude that accurate prediction of C18OO ecosystem fluxes requires careful representation of H218O and C18OO exchanges and transport in soils and plants.