Abstract The photocoloring and fading kinetics of the photochromic 3, 3-diphenyl-3H- naphtho [2, 1-b] pyran (NPY) have been analyzed using a kinetic treatment for three related species recently developed by us (Int J Chem Kinet 2005, 37 (3), 162). The concepts of identifiability and distinguishability are developed with respect to the elucidation of the NPY kinetics. We find that neither set of kinetic data on its own, nor the two data sets combined, ...