In SEC-FTIR, polymer separation according to molar mass, size exclusion chromatography (SEC) is coupled to a Fourier transform infrared (FTIR) analysis. Over the past few years, much effort was put into the determination of the chemical composition of a broad range of polyolefins as a function of molar mass using this technique (EPDM, LDPE, LLDPE). In the current investigation, a pressure transducer is used to optimize the SEC-FTIR technique for the analysis of semi-crystalline materials like HDPE and PP. It was shown that the comonomer content (butene for HDPE and ethene for PP) decreased with increasing molar mass. For two HDPE samples which were produced by the dual reactor process this decrease occurred step wise with a constant comonomer amount on either side of the step. For all HDPE samples it was shown that the amount of end unsaturations decreases with molar mass. The concentrations of trans and side unsaturations were too low to be determined.