Isotopic variations of snowmelt provide important information for understanding snowmelt processes and the timing and contribution of snowmelt to catchments in spring. We report a new model for simulating the isotopic evolution of snowmelt. The model includes a hydraulic exchange between mobile and immobile water, and an isotopic exchange between liquid water (mobile and immobile water) and ice within a snowpack. Since this model is based on the mobile-immobile water conceptualization, which is widely used for describing chemical tracer transport in snow, it allows simultaneous simulations of chemical as well as isotopic variations in snowpack discharge. We also report temporal variations of isotopic composition of a snowpack and snowmelt during artificial rain-on-snow experiments and diel snowmelt cycles observed in spring 2003 at the Central Sierra Snow Laboratory, California. These observations are used to test the newly developed model and to understand physical processes in a seasonal snowpack. Our model simulates the isotopic variations reasonably well, and suggests that exchanges of ice with both mobile and immobile water are important for determining the isotopic composition of the discharge.