Summary: The use of High Temperature Gel Permeation Chromatography (HT-GPC) with online infrared (IR) detector to analyze copolymer composition of propylene-1-butene (P-B) copolymer is investigated in this study. This technique is based on the measurement of the methyl to methylene ratio by an online infrared detector. This measurement is made possible by the highly discriminating power of a modern multiple band filter-based IR detector (IR5 MCT) to detect the P-B copolymer chemical structure. Also discussed is an alternative method, based on the application of the triple-detector GPC (GPC with IR, viscometer and light scattering coupled detectors) approach and Mark-Houwink plot.