The rate and equilibrium constants involved in the network of reactions of the 4'-hydroxynaphthoflavylium ion in moderately acidic and neutral media were characterized by means flash photolysis, stopped-flow and UV-vis absorption techniques. At the equilibrium, the network behaves as a two components system involving the naphthoflavylium ion and the trans-chalcone which is the thermodynamically stable species above pH = 2.9. All the rate and thermodynamic constants are very similar to those previously reported for 4'-hydroxy. avylium except the rate constant of the cis-trans isomerization which is much higher for the naphtho. avylium derivative. The lack of cis-trans isomerization barrier can be explained by the higher delocalization of the double bond between C(3) and C(4) charge through the naphthalene ring. Continuous irradiation experiments were carried out at the pH region that gives the higher photochromic efficiency.