The photooxidation of methyl vinyl ketone MVK, one of the most important products of isoprene that is emitted by plants, was investigated in the atmospheric simulation chamber SAPHIR for conditions found in forested areas. The comparison of measured trace gas time series with model calculations shows a gap in the understanding of radical chemistry in the MVK oxidation scheme. The possibility of unimolecular isomerization reactions were investigated by means of quantum-chemical calculations.