Since highly selective photocyclic aromatization (SCAT) of poly(substituted acetylene) s yields quantitatively the corresponding 1,3,5-trisubstituted benzene derivatives, the SCAT products were applied for quantitative determination of microstructures of copoly(substituted acetylene) s by NMR (nuclear magnetic resonance). By this SCAT-NMR method for two copolymers of two different types of substituted acetylenes, quantitative analysis for microstructures, i. e., triad distributions and compositions, have been achieved successfully under mild conditions, although they could not be obtained from the NMR of the original copolymers because of their broad and overlapped peaks.