A method for the determination of cellulosic-fibre properties, such as, but not limited to, residual lignin content of chemical pulp, with the aid of a spectroscopic technique obtained over a range covering the visible and the near-infrared regions of the electromagnetic spectrum, comprising the steps of obtaining a sample from the process line, minimally removing some excess water, exposing the fibres in the sample to a large beam light source, optionally moving the sample at a constant speed, and acquiring the spectral data over a pre-determined length of time, and correlating the spectrum to a previously determined calibration so as to determine the Kappa number. Alternatively, as an arrangement for process control, along with other varying configurations, the instant invention includes the mounting of the excitation light source and fibre-optic probe along the various locations of the kraft cooking and bleaching process, such as 1) at the drum brownstock washers and 2) throughout the bleach plant deckers, located just before the doctored blade, the probe being connected to a fast scanning spectrometer, via fibre optic cables, with a computer sequencing spectral acquisition and correlating the spectrum against a predetermined calibration, and logging the results. The process configuration, with the use of the instant invention, overcomes all limitations of prior art, including sampling hardware and sample preparation, and provides true online Kappa number determinations of the order of seconds, as compared to current state of the art Kappa analyzers which require 10 to 15 minutes per analysis. The present method is capable of measuring residual lignin content of chemical pulps with varying consistency, accounts for species effects to provide species insensitivity and can provide analysis in less than 10 seconds. The method can provide true online continuous measurement of Kappa number for feedforward and feedback control of the bleaching, paper machine and kraft cooking processes.