AbstractPolydiacetylene (2) of varying molecular weight has been formed by UV‐ or60Co γ‐irradiation of the diacetylene PhCH2CO2(CH2)9‐CCCC(CH2)9O2CCH2Ph (3), with conversion of 30−>95%. Samples of (2) with differing physical properties have been studied, in the bulk phase, by CPMAS13C NMR and VIS/UV spectroscopy and by DSC; complementary spectroscopic data on solutions of (2), including fluorescence studies, are also reported. Spectroscopic studies support the presence of an extended planar conjugated backbone in ‘as polymerised’ blue–black samples of (2), from which various forms of red‐coloured polymer, with shorter conjugation lengths, can be obtained. Studies of the thermal behaviour indicate the existence of several differing thermal transitions for these materials and the