Polyion complex of stearic acid-d(35) (St-d(35)) and 4-octyl-4'-(5-carboxypentamethyleneoxy)azobenzene (8A5) with polyallylamine has been prepared, and orientational regulation of St-d(35) by trans-cis photoisomerization of 8A5 in the Langmuir-Blodgett films has been studied by means of infrared reflection absorption (RA) spectroscopy. The RA spectra revealed that irradiation with UV light in the temperature range 45-65 degrees C leads to an orientation change in St-d(35) by 8A5 isomerization from the trans form to the cis form, and that irradiation with visible light returns 8A5 and St-d(35) to the initial states. However, below 35 degrees C and above 75 degrees C, the performance of the photoregulation of St-d(35)'s orientation considerably decreased.