Abstract: The present invention relates to a multiple catalyst system for preparing unsaturated aldehydes and acids from reactive hydrocarbons at high reactive hydrocarbon space velocity conditions. The present invention also relates to a process for preparing unsaturated aldehydes and acids from reactive hydrocarbons using the multiple catalyst system at high reactive hydrocarbon space velocity conditions. In one embodiment, the multiple catalyst system is utilized in a vapor phase catalytic oxidation reaction process which produces acrolein and acrylic acid from propylene at high reactive hydrocarbon space velocity conditions.