AbstractFive polyamine fixing agents with different molecular weights but slightly different charge densities were used to treat a deinked pulp. Their efficacy in controlling colloidal substances (CS) in the pulp was measured using focused beam reflectance measurements (FBRM). The objective was to determine if the colloidal substances were affected by the fixing agents by a “colloidal fixation” mechanism, i.e., colloidal particles being fixed onto pulp fiber in an un-agglomerated, single-particle state, or a “colloidal agglomeration” one, i.e., colloidal particles being coagulated by fixing agents into bigger agglomerates. The results showed that colloidal fixation does take place, especially for the polyamine with the smallest molecular weight. Among the five polyamines, it was found that higher molecular weights tended to result in more extensive colloidal agglomeration, but the effect of charge density was almost insignificant. Because it is efficient in differentiating between these two fixation mechanisms, FBRM is a powerful tool in screening different fixing agents.