Brittle intergranular cracking was observed on a medium carbon, matensitic steel subjected to sustained loading in air at 185-500°C. The slow crack growth rate increased with temperature in the lower temperature regime with an apparent activation energy of 68.5 kJ/mol. It dropped down rapidly after reaching its maximum at ∼400°C. No slow crack growth was observed above 500°C. Control tests at 300°C in argon and nitrogen gases indicated that the presence of oxygen enhanced the slow crack growth rate. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) revealed enriched carbon and oxygen all the time and sulfur occasionally on the fractured intergranular facets. The latter was shown likely to be due to free surface segregation and thus not responsible for the embrittlement. Based on AES/XPS analysis and kinetic considerations, it is proposed that a synergetic effect of carbon and oxygen led to the observed embrittlement phenomenon. The kinetic behavior of the embrittlement is modelled based on the diffusion and collection of carbon in the process zone.