Nitrosobenzenes add to 2-substituted butadienes to give, in most cases, 4-substituted 3,6-dihydro-1,2-oxazines. The structure of these adducts is proved by NMR-spectroscopy and chemical means. The kinetics of the Diels-Alder reaction of 2-arylbutadienes with p-chloro-nitrosobenzene has been studied.