Crystal violet lactone (CVL, 1, 3,3-bis[4-(dimethylamino)phenyl]-6-(dimethylamino)-1(3H)-isobenzofuranone) and other lactone dye precursors 2-5 form colored triphenylmethyl cations when irradiated with UV light in the presence or absence of [4-(octyloxy)phenyl]phenyliodonium hexafluoroantimonate (OPPI). In selected monomers in the presence of iodonium salt, color formation and polymerization occur simultaneously. The excited singlet state lifetimes of 1-5 estimated from quantum yields for the disappearance of CVL in the presence of OPPI are similar to those observed for fluorescence quenching and occur mainly by electron transfer. On the basis of the transient absorption spectral it can be concluded that triarylmethane cations are generated from the lactone precursors in the presence of OPPI in acetonitrile with a pseudo first order rate constant of 10(4) s(-1). Color formation in the absence of OPPI is suggested to occur from the excited singlet states of the lactone via a biradical formed by photoinduced beta-bond cleavage.