Photochemical Primary Processes of Xanthene Dyes. I. Investigations of the Primary Processes of Selenopyronine by Flash Excitation The triplet‐state of selenopyronine absorbs light in the whole investigated spectral range (λmax = 400 nm, 480 nm, 690 nm). As results of the bimolecular triplet decay a half‐reduced (λmax = 430 nm) and a half‐oxidized (λmax = 475 nm) form of the dye are observed. p‐Benzoquinone quenches the triplet‐state (k7 = 1,5 · 109 l/mol s) and the results are the half‐oxidized form and the p‐benzosemiquinone ion. For these two products different decay processes exist. The reducing agents DABCO, EDTA and Hydroquinone also quench the triplet state (k10 = 1,2 · 106 l/mol s, k11 = 1,0 · 106 l/mol s, k12 = 1,0 · 109 l/mol s) and as result the half‐reduced form is observed. Measurements with thiopyronine give analogous results, which are in good agreement with investigations published in the literature.