Microstructures in the copolymer of 1-chloro-1,3-butadiene (CB) and styrene (St) obtained by radical polymerization in bulk were investigated by spectroscopic methods. The microstructure of CB units in the copolymer was found as follows: 60% 1,4 and 40% 3,4 structures by 1H NMR; ca. 50% trans-1,4, ca. 5% cis-1,4, and ca. 45% 3,4 structures by IR. About 60% chlorine in the 1,4 CB unit was estimated to be very reactive because it is an allylic chlorine. After hydrogen chloride elimination from poly(CB-co-St), diad(1,4 CB-1,4 CB) and triad(1,4 CB-1,4 CB-1,4 CB) were determined by UV-visible spectroscopy. The results were in accord with the theoretical ones calculated from the monomer reactivity ratios derived from the terminal model (simple Markov) and the feed composition; the distribution of 1,4 and 3,4 structures was assumed completely random (Bernoullian).