The photochromic reactions of 6-phenyloxy-5,12-naphthacenequinone ( 1) and of the 6,11-diphenyloxy derivative 2 were investigated by subpicosecond pump-probe, photoacoustic, and emission spectroscopies, and by nanosecond laser. ash photolysis ( LFP). The transformation of the trans-quinones 1 and 2 to their ana-isomers proceeds via short-lived triplet states of 1 and 2 ( tau ca. 2 ns) and spiro-bridged biradical intermediates ( ca. 6 ns). The long-lived ( mu s) ana-triplets that are observed by LFP of 1 and 2 are formed ( predominantly) by reexcitation of the biradicals and ana-quinones, which appear during the laser pulse. The reverse reaction, ana -> trans, proceeds exclusively from the lowest pi, pi* singlet state of the ana-quinones.