Highly oxygenated organic molecules by atmospheric oxidation of plant emitted monoterpenes are important components in secondary organic aerosol formation. Autoxidation of organic peroxy radicals is one important pathway of their formation. We show that isomerization of highly oxygenated alkoxy radicals leads to highly oxygenated peroxy radicals that continue the autoxidation chain. Alkoxy-peroxy steps may dominate the formation of highly oxygenated molecules at high nitrogen oxide levels.