Abstract: A method for the on-line analysis of a process stream, which process stream is a feedstream to or an exit stream from a steam reformer, which process stream has a temperature of at least 200° C., the components of which process stream are in the vapour phase, which method comprises: (a) taking a slipstream from the process stream; (b) cooling the slipstream to a temperature above its dew point; (c) analysing the cooled slipstream by near infra-red (NIR) spectroscopy to obtain a spectrum characterising NIR-absorbing components of the process stream; and (d) correlating the spectrum obtained to established calibration models from NIR spectroscopy using chemometric techniques to determine the concentration of, and/or to determine the partial pressure of one or more of the NIR-absorbing components of the process stream.