The invention relates to a method for investigating process streams comprising five or more different hydrocarbon-containing components. In the method at least one process flow line (35) is in operative connection with an online IR spectrometer (2) and an online gas chromatograph (1). The process stream passed through the process stream conduit (35) is subjected to an online characterization which comprises measurements both with the online IR spectrometer and with an online gas chromatograph. The spectral data and the chromatography data are mathematically related to one another by suitable statistical models, thus allowing training of a model used for evaluating the analytical data and for characterizing the process streams. The method according to the invention is characterized by short measurement times in the range of seconds and milliseconds and a high accuracy. The method according to the invention for investigating process streams preferably relates to investigation of process streams deriving from processes proceeding in parallel, the process streams preferably deriving from reaction spaces arranged in parallel.