Unstressed coupons of Alloy 600 SA (solution annealed) and Alloy 600TT (thermally treated) were exposed to simulated pressurized water reactor (PWR) primary water at 360 °C. The depth of intergranular (IG) oxide penetration as well as the microstructure and microchemistry associated with IG oxidation of selected samples were characterized by electron microscopy and atom probe tomography techniques. There is no clear trend to indicate that IG oxidation depths in unstressed coupons are different between Alloy 600 SA and TT. At the same time, all SA+TT materials examined evidenced significant interaction between oxidation and Cr-rich grain-boundary (GB) precipitates. In addition, the spatial extent of GB compositional changes ahead of the oxidation front (e.g., Cr depletion and Ni enrichment) appears different for Alloy 600 SA vs. SA+TT. In SA materials exposed for only 1000 h (42 days), the composition of some GBs changes over distances of 500–1000 nm. SA+TT materials appear not to develop such long-range GB compositional changes (extending over 10s of nm).