Small addition of potassium to a Mo-V-Sb mixed oxide catalyst (previously prepared by hydrothermal synthesis) strongly modifies its catalytic behavior. Thus, while acetic acid is mainly observed in the K-free catalysts, acrylic acid is selectively obtained in K-doped catalysts. In this case, a selectivity to acrylic acid of about 30% is achieved at propane conversions of 30%. This catalytic behavior is apparently not due to modi. cation of the crystalline phases in the K-doped catalysts but to the elimination of the acid sites of the undoped Mo-V-Sb mixed oxide catalyst.