Thermal behavior of a Langmuir-Blodgett (LB) film of dioctadecyidimethylammonium-Au(dMit)(2) (2C(18)-Au(dmit)(2)) salt has been studied by means of infrared transmission spectroscopy. Two bands due to the CH2 antisymmetric and symmetric stretching modes have been used to monitor the orientation of the hydrocarbon chain in the 2C(18)-Au(dmit)(2) LB film, and the absorption band due to the C = S stretching mode has been employed to investigate the aggregation state of Au(dmit)(2) chromophores. Temperature-dependent infrared spectral changes have revealed that there are, at least, two different ordered states in the LB film below the order-disorder transition between 100 and 110 degreesC. The above results are important, showing that the heat treatment can be a powerful and versatile tool to control the structure of the 2C(18)-Au(dMit)(2) LB film. (C) 2004 Elsevier B.V. All rights reserved.