Cleaved MgO was subjected to fluid and gaseous chemical attack by water at or near ambient temperature. The exposure experiments were designed to mimic the variety of conditions that MgO may encounter as a substrate material in thin film technology. The resultant surface alterations were monitored with contact mode atomic force microscopy. It was found that MgO was relatively inert, at the near-atomic scale of detail, when the relative humidity of the gaseous phase was below 30%. When the humidity exceeded 30%, degradation was initiated at sites of low coordination (corners and steps). These results would indicate that, when handling MgO as a substrate for thin film technology, the processing environment must be carefully controlled to exclude bulk water and maintain humidity below 30%.