Slice inverses of temperature and heat content from the 1988--1989 Greenland Sea Tomography experiment and other observations, including standard conductivity-temperature-depth stations, moored thermistors, surface meteorological variables, and surface ice cover are combined to better understand the thermodynamics of the Greenland Sea Gyre. Thermal evolution of the gyre center seems to divide naturally into the following three periods: a preconditioning phase (November--January), during which surface salinity is increased by brine rejection from ice formation and by entrainment but in which the mixed layer deepens only slowly to a depth of some 150--200 m; a deep mixing phase (February--March) during which the surface mixed layer deepens rapidly to approximately 1500 m in the gyre center purely under the influence of local surface cooling; and a restratification phase during which the products of deep mixing are replaced by Arctic Intermediate Water flowing in from the gyre edges. The onset of the deep mixing phase occurs after ice formation in the gyre center stops, resulting in an area of open water where large heat fluxes can occur. In surrounding regions, including the Odden region to the south, ice is still being formed. To the north and west, closer to the steep topography of the continental shelf, the inverse results show significant variability due to advection, and large temperature and heat content fluctuations with a period of about 50 days are seen. ¿ American Geophysical Union 1995