Diene groups in the terminal position of the lipid acyl chains undergo an irreversible cis --> trans isomerization when lipid films are incubated on a solid support under an atmosphere which is adjusted to a relative humidity (RH) in the range 20% < RH < 100%. In general, isomerization and polymerization can be modulated by RH, temperature, and the phase state of the lipid. Under nearly dry conditions or in water excess the isomerization is paralleled by the spontaneous polymerization of the diene groups. Alternatively, polymerization can be realized by UV irradiation. This process is accompanied and/or preceded by the cis --> trans photoisomerization of the terminal diene groups. The kinetics of isomerization and polymerization are investigated by means FTIR spectroscopy using selected absorption bands of the diene groups. The lipid films are characterized before and after polymerization. Polymerization of solid lipid phases destroys the crystalline packing of the acyl chains and causes conformational defects between the methylene units. Nevertheless it gives rise to polymerized multilayers which are well-oriented on the solid support. The analysis of the kinetic data gives rise to the conclusion that the degree of molecular ordering within the hydrophobic region of the lipid aggregates controls the isomerization and polymerization reactions. Both processes are assumed to proceed via an intermediate twisted conformation.