Abstract: Method for predicting properties of a product of cellulose-fibre-based pulp, paper or paperboard, wherein a suspension of cellulose fibre is subjected to analysis, during manufacture of the product, by spectroscopic measurements of a sample quantity of the suspension, in a spectrum in the wavelength range 200–2500 nm. The method comprises diluting each sample quantity prior to analysis, dewatering and drying one partial flow and subjecting that partial flow to spectroscopic measurements, and analyzing a second partial flow of the diluted sample quantity for physical fiber data, by means of image analysis. Each sample quantity analyzed generates at least 100 data points from the spectroscopic measurement distributed in the selected spectrum, and at least 50 data points with regard to the physical fibre data, in the form of one or more physical fibre property distributions with regard to at least one physical property selected from fibre length, fibre width and fibre shape.