Magnetic thin films of Co80Cr16Ta4 were sputtered onto identical CrMo seed-layers at –200 V bias and 3 different substrate temperatures (150, 200, and 250 °C). Energy-filtered transmission electron microscopy (EFTEM) was performed to analyze Cr levels at the grain boundaries as well as inside the grains. These quantitative Cr measurements were used to estimate the local values of magnetocrystalline anisotropy (K u ) and, together with grain size distributions, calculate the product of K u and the grain volume (K uV ), a quantity which is a measure of thermal stability. The results show that the coercivity as well as the fraction of stable grains increased with increasing substrate temperature. The increase in the fraction of stable grains is produced by the enhancement in the K u value due to Cr depletion of the grain interiors and the magnetic decoupling between the grains from Cr grain-boundary segregation.