We have studied the effects of thermal annealing (between 240 and 270°C) on the magnetic and interface properties of exchange-biased NiFe/FeMn thin film couples. With moderate annealing, there is a change in the temperature-dependence of the exchange-bias from an initial, linear shape to a convex shape, which better approximates the asymptotic behavior set by the temperature dependence of the sublattice magnetization in bulk FeMn. With extended annealing, the magnitude of the exchange-bias at room temperature increases significantly, and the blocking temperature (where the exchange-bias vanishes) increases beyond the Neel temperature of FeMn. Auger composition analysis shows that after extended annealing, there has been substantial interdiffusion of Ni into the FeMn, Mn into the NiFe, and Fe away from the interface; the original NiFe/FeMn interface becomes preferentially enriched in Mn and Ni, and deficient in Fe. Extended annealing apparently leads to the formation of a NiMnFe alloy that exhibits strong antiferromagnetic order and couples to the NiFe to produce a much stronger exchange bias than the original FeMn alloy.