Research has been undertaken in which a combination of different techniques was utilised to compare pre-flocculation (flocculant added to the filler prior to its addition to the pulp) with pre-mixing strategies, also called co-flocculation (chemicals added to a mixture of cellulose microfibrils (CMF) and filler. The pulp used was a mixture of 80% hardwood kraft pulp and 20% softwood kraft pulp. The filler was ground calcium carbonate (GCC), while the chemicals used included cationic polyacrylamide (CPAM), silica, bentonite, anionic PAM (APAM), cationic starch and CMF. Using the focused beam reflectance method (FBRM) enabled an in-depth and quick study of the filler floc behaviour at different dosages and mixing conditions, demonstrating that it was a very good tool as a pre-screening for different treatment alternatives. Clear differences in behaviour between different pre-flocculation and pre-mixing strategies were observed. For the pre-mixing strategy, the most promising results were achieved with CPAM plus bentonite and cationic starch systems. Although dewatering was negatively affected with pre-mixing, the mechanical properties were clearly improved. Flocs obtained with PCAM/bentonite had a similar resistance with both strategies, while cationic starch was clearly more advantageous when using the pre-mixing strategy.