Thermal behavior of Langmuir-Blodgett (LB) films of 5-(4-N-octadecylpyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) was investigated by use of ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. With increasing temperature from 30 degrees C to 170 degrees C, Soret bands of one- and nine-layer LB films of porphyrin 338a on CaF2 substrates show a blue shift by 6 and 2 nm, respectively. In contrast, a Q(x)(0,0) band shows a red shift by 10 and 5 nm, respect-very, for the one- and nine-layer LB films. The one-layer LB film shows more significant shifts of the Sorer and Q(x)(0,0) bands than the nine-layer film, indicating different thermal behavior of the porphyrin aggregates, which may be induced by different effects of interaction between the substrate and the first monolayer, and by the existence and nonexistence of interlayer interaction. In the IR spectra of a nine-layer LB film of porphyrin 338a on a CaF2 plate, the frequencies of CH? antisymmetric and symmetric stretching bands increase significantly with temperature, suggesting the increase in gauche conformers in the hydrocarbon chain of the porphyrin 338a. In contrast, the frequencies of bands arising from the porphyrin macrocycle and attached pyridyl groups decrease gradually with the increase in temperature. These observations indicate deformation of porphyrin core and changes in coplanarity of the pyridyl groups with the porphyrin plane.