Samples of “sandstone” from near the site of the upper Rio Blanco nuclear explosion were heated in the laboratory at temperatures between 600 and 900°C. The composition and amount of noncondensable (dry) gas released were measured and compared to the amount and composition of gas found underground following the explosion. The gas released from the rock heated in the laboratory contained ∼80% CO2 and 10% H2; the balance was CO and CH4. With increasing temperature, the amounts of CO2, CO, and H2 released increased. The composition of gas released by heating Rio Blanco rock in the laboratory is similar to the composition of gas found after the nuclear explosion except that it contains less natural gas (CH4, C2H6 . . . ). The amount of noncondensable gas released by heating the rock increases from ∼0.1 mole/kg of rock at 600°C to 0.9 mole /kg at 900°C. Over 90% of the volatile components of the rock are released in <10 h at 900°C. A comparison of the amount of gas released by heating rock in the laboratory to the amount of gas released by the heat of the Rio Blanco nuclear explosion suggests that the explosion released the volatile material from about 0.42 mg of rock per joule of explosive energy (1700 to 1800 tonnes per kt).