Mirror γ-ray transitions can be split into a “strong” and a “weak” isospin component. It has been normal practice to assign E1 IV, E2 IS or M1 IV character to the strong, and E1 IS, E2 IV or M1 IS character to the weak component (IV = isovector, IS = isoscalar). It is argumented that this procedure should only be applied if the strength of the strong component exceeds the recommended upper limit for the weak component. As an illustration we discuss the 2 2+ → 0 1+ E2 transitions in 26Mg, 26Al and 26Si. Branching and lifetime of the 26Al, 3.16 MeV, 2 2+, T = 1 level have been measured with the 25Mg(p γ) 26 Al reaction. Both the 3.16 → 0.23 MeV 2 2+ → 0 1+ branching, (0.39 ± 0.03)%, and the lifetime, τm = 4.9 ± 1.2 fs, are smaller by an order of magnitude than the values known from previous work. The present values yield S(E2 IS) = 0.64 ± 0.17 Wu. From the known E2 strengths of the analogue transitions in 26Mg and 26Si their isovector component is then obtained as S( E2 IV) = 0.14 ± 0.08 Wu.