AbstractThe unsensitized photoinitiation of styrene polymerization is accompanied by the formation of some typical oligomeric key substances, which are indicative of two 4π + 2π‐photo‐intermediates (I), viz. 1‐phenyl‐1,2,3,8a‐tetrahydronaphthalene (1) and 2‐phenyl‐1,2,3,8a‐tetrahydronaphthalene (1′). Owing to the very different absorption coefficients of monomer and I, formation and consumption and, therefore, the photo stationary concentration of I exhibits a pronounced dependence on the wavelength λeof the light used for irradiation. Under conditions, where the exciting light is not homogeneously but totally absorbed in the system (λe= 291 nm, corresponding to the 0–0 band of the lowest1Lb←1A transition in styrene), the intermediates I can be enriched to such an extent, that they are detectable not only by UV‐absorption (310⩽/nmλ/nm ⩽ 360) but also—with the aid of a special high vacuum technique—by1H NMR spectroscopy, which gives unambiguous proof for the molecular structure suggested for I. Moreover, the photochemistry, the photokinetics and the photophysical behaviour of I were investigated in detail. In the absence of any fluorescence (φ FI<10−4) of the intermediate the overall quantum yield for consumption was found to be φ–1= 1. It is shown, that the photo‐retro‐Diels‐Alder process is the dominating reaction (φ2M= 0,94) and the 1,3‐H‐sigmatropic rearrangement does not contribute more than φPhT⩽0,06. Together with the mean natural molar absorption coefficientsφI= 18400 dm3·mol−1·cm−1for λ = 313 nm andφI= 7300 dm3·mol−1·cm−1for λ = 335 nm the photostationary concentration [I]∞is calculated to range from [I]∞= 8,7.10−6mol. dm−3at λe= 313 nm to [I]∞= 1,2.10−6mol.dm