A methodology has been verified where the ZD-distribution of a component of the furnish (fines or CMF) can bemonitored. The component to be monitored was dyed using a direct azo-dye before addition to the furnish. Theproduced sheets were sectioned in cross-thickness direction by sequential heat-seal lamination and splitting, usingplastic pouches. The amount of dyed material in a split was monitored using thea* and L* parameters of the CIELabcolor space and the opacity, all measured simultaneously with a spectrophotometer. The value on the green-redaxis,a*, the lightness, L, and the opacity were combined into a new parameter, -a*L/opac. Two mechanisms forfines retention was observed by laboratory studies, mechanical retention by filtering process in the formed fiber weband chemical retention by agglomeration to the fibers.The methodology has recently been applied for monitoring cross-thickness distributions of either CMF or fines inpaper produced in pilot scale trials. For example, it was demonstrated, that the machine settings affected themechanisms for retention of fines in the fiber web during sheet forming and dewatering, with different distributionsof fines in the cross-thickness direction as result.