Reaction centres (RC) isolated from Rhodobacter sphaeroides were arranged in Langmuir-Blodgett (LB) film. The deposited layers were investigated with a spectrophotometric assay after heating at different temperatures up to 473 K. The examination of the optical properties of RCs gives a first indication of the molecule stability in the film after temperature treatment. Comparisons of LB absorption spectra between film and solution after treatment at different temperatures were carried out. The absorbing bands of Bacteriopheophytin (Bphe), Bacteriochlorophyll (BChl) and Bacteriochlorophyll dimer (BChl(2)) were investigated. The results indicate that RCs which in the solution appear stable up to 318 K maintain their absorption spectrum up to 358 K inside the film. The role of LB organization was checked by measuring the optical properties of non-oriented films after heating at different temperatures.