The electronic spectra, kinetic regularities, and the mechanism of decay of the cis and trans isomeric forms of 4-[(2E)-1-methylbut-2-en-1-yl]phenylnitroso oxide (2) were studied by flash photolysis and product analysis. The mechanism of the consumption of this nitroso oxide is the same as the one proposed earlier for 4-methoxyphenylnitroso oxide. The trans-2 isomer is converted into cis-2, which undergoes cyclization to the substituted benzo[d][1,2,3]clioxazole 3. The reopening of the dioxazole ring yields nitrile oxide 4. The final product (3,4-dimethyl-3a,4-dihydro-2,1-benzisoxazol-5(3H)-ylidene)acetaldehyde (5) is formed by the intramolecular [3 + 2] -cycloaddition of the nitrile oxide group of 4 to the allylic double bond. To support the proposed mechanism, the quantum chemical calculations have been employed.