UV-induced polymerization of thin (1-4 monolayers) Langmuir-Blodgett films of acetylenic carboxylic acids with triple bonds in different positions (terminal: 23-tetracosinic acid HC C(CH2)21COOH, and internal: 2-docosinic CH3(CH2)18C CCOOH) and their lead salts is investigated. It is shown by means of IR spectroscopy that the topochemical reaction proceeds with the participation of carboxylic groups. The differences in the structure of mono- and bilayers are demonstrated. The mechanism of the topochemical reaction depends on the method of film transfer onto the substrate. It is shown by means of UV spectroscopy that short conjugated polyenes (containing 7 to 9 carbon atoms) are formed as the product of polymerization. The mechanism of the formation of these polyene chains is proposed on the basis of the experimental data and semi-empirical calculations. (C) 2009 Elsevier B.V. All rights reserved.