Surface plasmon resonance (SPR) properties were measured for porphyrin Langmuir-Blodgett (LB) films and the response to NO2 gas was investigated. The porphyrin molecule was 5,10,15,20-tetrakis(3,4-bis[2-ethylhexy-loxyphenyl])-21H, 23H-porphine (EHO). The EHO porphyrin LB films were deposited onto the cover glass with evaporated Ag thin films by vertical dipping method with a very fast deposition rate of 1000 mm min(-1). The SPR measurements were carried out at wavelengths of 488 and 632.8 nm. The SPR properties measured at 488 nm were considered to be related to the dispersion properties due to the optical absorption band of the EHO LB films. The thickness and the complex dielectric constants of the EHO LB films evaluated from the SPR properties measured at 632.8 nm were thought to be related to the island structure of the EHO LB films. From the NO2 gas response measurements, the SPR properties at 488 nm were found to be more sensitive to NO2 gas than those at 632.8 nm. The response rate to NO2 gas and the recovery properties were also examined. (C) 2002 Elsevier Science B.V. All rights reserved.