The process for oxidation of propylene to acrolein and the oxidation of propylene to acrylic acid in two stages with acrolein as an intermediate are improved by use of essentially inert essentially anhydrous diluent gases to replace steam in the reaction streams. In particular, the use of essentially inert essentially anhydrous diluents which raise the composite heat capacity of the diluent gas mixture to at least about 6.5 calories/(gram-mole) (.degree.C.) will improve selectively to desired products and will reduce both the waste water load on the system and by-product formation. Reduction in second-stage catalyst efficiency is compensated for by introducing steam to the feed to the second-stage, and/or by increasing the operating temperature of the second-stage, and/or by increasing the active surface area of the second-stage catalyst.