Immediate gas changes occurred when untreated Martian surface samples were humidified and/or wet by an aqueous nutrient medium in the Viking lander gas exchange experiment. The evolutions of N2, CO2, and Ar are mainly associated with soil surface desorption caused by water vapor, while O2 evolution is primarily associated with decomposition of superoxides inferred to be present on Mars. On recharges with fresh nutrient and test gas, only CO2 was given off, and its rate of evolution decreased with each recharge. This CO2 evolution is thought to come from the oxidation of organics present in the nutrient by &ggr;Fe2O3 in the surface samples. Atmospheric analyses were also performed at both sites. The mean atmosperic composition from four analyses is N2, 2.3%; O2, ?0.15%; Ar, 1.5%; and CO2, 96.2%.