The selective gas phase oxidation of o-xylene was investigated inside of the explosion regime using a microstructured reactor. The V2O5/TiO2 catalyst was used as microfixed bed and as catalyst coating. There were no significant losses of the selectivity to phthalic anhydride with feed compositions up to 7vol% o-xylene observable. Above 7vol% the selectivity decreased due to total oxidation, especially for the microfixed bed, which was probably caused by a hotspot.