Transient species upon pulsed laser excitation of ultrathin Langmuir-Blodgett (LB) films was detected sensitively by the optical waveguide (OWG) method. Amphiphilic tetraphenylporphyrin in mixed LB films with dioctadecyldimethylammonium at 1:5 and 1:50 were prepared on the surface of OWGs. These films were excited with the second harmonics of a ns Nd:YAG laser in degassed environment. Transient absorption of excited triplet porphyrins in LB films with only two monolayers was detected by the present method. It was about hundred times increase of sensitivity as compared with a conventional normal incidence measurement. Different decay dynamics were observed for excited triplet state porphyrins in LB films depending on the molar ratio, which was attributed to inhomogeneous distribution of chromophores.