Pyrite exists in accumulations of finely divided grains or crystals in many types of specimens. When such specimens are exposed to air the pyrite (ferrous disulphide) is oxidized to ferrous and ferric sulphates. This results in a large volume increase and consequent rupture and crumbling of specimens. A study of pyrite oxidation at intermediate relative humidities is being undertaken to provide date on the effect of RH on the rate at which oxygen is consumed and oxidation products are formed. These data may provide a basis for recommending RH levels appropriate for safe storage of pyritic materials, etc. Test methods are described. Early results show, among other things, that initial oxidation rates of freshly ground crystalline pyrite increase exponentially as RH increases from 10 percent to about 60 percent. Further experimentation is planned.