Mixed Langmuir monolayers of 10,12-Pentacosadiynoic acid (DA) monomer and an amphiphilic Hemicyanine dye derivative have been formed at the air/water interface. Two derivatives of docosylpyridinium have been used, with either one included in the monolayer in 1:1 molar ratio. The DA monomers within the mixed monolayers have been polymerized in situ at the air/water interface. The crystalline structure of the monolayer and the kinetics of polymerization have been probed grazing incidence X-ray diffraction (GIXD). The polymerization of DA proceeds with no phase segregation, exclusively leading to the red polydiacetylene form. The kinetics of polymerization at the air/water interface has been monitored in situ by GIXD. The experimental results have been combined with Molecular Mechanics computer simulations, revealing that DA molecules are sequentially arranged with molecules of Hemicyanine dye in alternating rows. The hydrophobic chains of the dye molecules act as spacers between the DA monomers. Surprisingly, such molecular arrangement does not hinder the in situ photopolymerization of DA. The mechanism of polymerization of DA within the mixed Langmuir monolayers has been convincingly described in molecular detail. This approach for interfacial polymerization of DA holds great potential for optically active devices and nanostructures comprising self-assembled thin films based in polydiacetylene.