A low-viscosity 60 SnO-40 P2O5 (mol%) glass was reheated at 280 degrees C (about 45 degrees C above the glass-transition temperature) for 20 min in various atmospheres (Ar, air, and O-2), then the structure- and surface-related properties were examined. It was found that increase in Po-2 increases surface hardness, reduces optical transmittance, and improves chemical durability. The above phenomena are explained in terms of the increased oxidation tendency of Sn2+ to Sn4+ on the glass surface during reheating in increased P-O2.