In order to study the structure of the chlorinated IPP''s obtained, IR spectroscopic investigations were carried out: 1. A tendancy about the reactivity of the three different kinds of protons in IPP structure unit obtained by means of detailed interpretation of C-Cl streching vibration absorption bands was observed that methylene protons might be more reactive to chlorine than methyle or methine protons. 2. The intensity ratio values for the IR absorption bands of 990 cm^-1/974 cm^-1were found to be directly proportional to the degree of remaining crystallinity, which was decreased rapidly with higher chlorine content. 3. The very small amount of oxygen present in the reaction system brought about formation of abnormal structures like C=C and C=0 double bonds on the polymer chain and simultaneously resulted in a great decrease in molecular weight, which was increased inversely at chlorine content higher than 35V%. In this case the possibility of cross-link was present.