Noteworthy, the even more substituted 2,5-dimethyl-2,4-hexadiene, where the T1 state would be described as composed of a tertiary alkyl radical and a terminally dimethyl substituted allyl radical, has been reported to not form dimers when the sensitized photodimerization is attempted. This supports our observation of a relationship between the reactivity of 1,3-dienes in these photoreactions and the extent of methyl substitution at or near the reactive C atom of the alkyl radical moiety. Isoprene (2) in T1 is the most reactive diene because it is the mono-methyl substituted diene with a reactive alkyl radical moiety being a primary alkyl radical. Its isomer, piperylene (4), is less reactive as its most stable conformer in the T1 state has a secondary alkyl radical moiety. The dimethyl substituted dienes are all considerably less volatile and/or have less reactive secondary alkyl radical moieties.