White light optical waveguide (WOWG) method was applied to measurement of transient absorption spectra of ultrathin polymer and Langmuir-Blodgett (LB) films. Tetraphenylporphyrin derivatives in poly(methyl methacrylate) films and in a mixed LB films with arachidic acid were prepared on the surface of OWG glasses. The WOWG method has two orders of magnitude higher sensitivity than the conventional absorption method. The transient absorption changes of intramolecular electron transfer reactions between porphyrin and viologen were detected highly sensitively in ultrathin films by the WOWG method.