The present investigation has shown that the use of image analysis can be a very efficient way to study the state of flocculation of cellulosic fibres. By an FFT analysis of the grey-values of the collected images, of the flowing fibres, it was possible to evaluate both a degree of flocculation of the fibres and an average diameter of the formed flocs. Calibration with model flocs correlated very well with earlier published data.Investigations regarding flocculation of unbleached cellulosic fibres caused by a microparticle based retention aid, i.e. a combined addition of a high molecular mass cationic polyacrylamide (C-PAM) and a bentonite clay, and a site blocking polymer (SBP) (a dimethylamine epichlorohydrine condensate) were also conducted. The results from these measurements show that a preadsorption of the SEP to a surface coverage of 50% of the fibres (i.e. of its saturation adsorption) resulted in a higher degree of flocculation upon addition of the C-PAM of the microparticle based flocculant. This is probably due to a more extended conformation of the C-PAM out from the surface of the cellulosic fibers. These results were also in accordance with earlier published theories. A simultaneous addition of SEP and C-PAM in the microparticle system was not as efficient, regarding flocculation, as a pre-adsorption of the SEP. This can be explained by the fact that the way that both the SEP and the C-PAM are adsorbed is affected by the way the polymers are added. In accordance with earlier publishd data the results indicate that the adsorption of SEP is strongly affected by the presence of the C-PAM.The results nevertheless show that an addition of a SEP has a positive effect on microparticle based retention aids.Finally the present results clearly demonstrate that the polymer addition levels in flocculation studies (which are relevant for practical use) are comparably low compared with saturation adsorption on the fibres and the difference in flocculating ability of differently charged C-PAM is small, compared with their corresponding saturation adsorption. Further investigations in this area are strongly recommended.