Absorption, infrared reflection absorption (IRA), and resonance Raman scattering (RRS) spectroscopies were applied to obtain direct information about the conformation change of a polydiacetylene (abbreviated to PDA) produced by UV irradiation of the Langmuir-Blodgett (LB) films of the cadmium salt of a diacetylene derivative, 10,12-pentacosadiynoic acid (DA), with the number of monolayers from 1 to 31. The rate and extent of the polymerization of DA monitored by the relative intensity change of the IRA bands due to C=C stretching vibrations were correlated with orientation changes of the alkyl groups of DA and changes in the conjugation length of the PDA backbone elucidated by RRS spectra in the C=C stretching vibration region. The results proved the following mechanism of the conversion from a blue (lambda(max) approximate to 635 nm) to a red phase (lambda(max) approximate to 540 nm) during the polymerization of DA. The conversion proceeds through two steps, The polymerization reaction itself is almost completed in the first step, although a residual reaction may take place in the second step. During the first step the plane formed by the all-trans zigzag backbone of the alkyl groups changes its arrangement in the following way : the orientation of the long axis of the alkyl backbone changes from a tilted state to a less tilted one, keeping the axis perpendicular to the long axis within the plane parallel to the substrate surface. This orientation change conforms to an increase in the interlayer spacing of the LB films of long-chain diacetylene monocarboxylic acids during their photopolymerization process observed through a small angle X-ray diffraction technique (Lieser, G.; et al. Thin Solid Films (1980, 68, 77). During the first step the PDA backbone keeps a fully extended conformation without any interruption of its conjugation, and the formation of the blue phase proceeds, When the polymerization proceeds to the second step, the regular plane of the all-trans alkyl group is converted to an irregular one containing gauche conformations, This conversion causes an interruption of the fully extended backbone structure and a reduction of the average conjugation length, resulting in the formation of the red phase.