The chlorophyll (chi) alpha-cytochrome (cyt) c mixed system was studied in Langmuir films at the air-water interface and Langmuir-Blodgett films. Surface pressure-molecular area isotherms measured for pure and mixed components were discussed in terms of ideal mixing processes. The negative deviations from ideality observed at cyt c molar fractions lower than 0.1 can be interpreted as a result of pigment-protein complexation. On the other hand, at protein molar fractions higher than 0.1, progressive positive deviations were observed and could be due to cyt c aggregation in the mixed system. Furthermore, the spontaneous complexation of the chi a and cyt c (in solution) was observed in hydrated ethanol (water:ethanol 97:3). In this case, the protein molar fraction was found to be 0.2. Langmuir-Blodgett films of the chlorophyll-protein mixtures were also analyzed using absorption, Fourier transform infrared, and fluorescent spectroscopic techniques. The infrared spectroscopy showed that at protein molar fractions higher than 0.1, cvt c molecules undergo drastic conformational changes from alpha-helix to beta-sheet and turn structures. Such conformational changes are interpreted in terms of protein aggregation and denaturation. Spectroscopic evidence has also indicated the participation of the keto group in the chlorophyll-protein interaction and the presence of chlorophyll molecules in an aggregated form in the complexes.