Aluminium sulfate (alum) has been used as the rosin sizing precipitant for along time, but it is only valid in acid pH. With the rapid development of paperindustry, the neutral sizing has shown more and more benefits and tends toreplace the acid sizing. The neutral sizing with anionic dispersed rosin size (DRS)and polyaluminium chloride (PAC) instead of aluminium sulfate makes full useof the cheaper rosin and can be achieved without changing the former acidsystem too much. Being practical and cost-effective, this method is especiallysuitable to our country. The following aspects were studied centering on theneutral sizing system composed of PAC and DRS.Firstly, the characteristics of PACs and the relative factors were studied. It isshown that PACs are formed by hydrolysis and polymerization and thealuminium species are fairly complicated. They are affected by total aluminiumconcentration and preparing methods as well as basicity mainly in differentcomponents of Ala, Alb and Alc. In addition, the aluminium species component isalso affected by some environmental conditions, such as diluting in water,different pH value, aging time and so on. When diluted in water, PACs will bedifferent on aluminium species along with the stages and times of dilution. WhenpH value is increased or aging time is postponed, the aluminium species of PACswill convert to the higher polymer species.Secondly, both of the practice of PAC in wet end and the comparison of PACwith alum in different aspects were studied in detail. The results fromexperiments indicate that different kinds of PAC as rosin sizing precipitant haveshown different contribution to sizing degree. Some cationic additives such asamphoteric starch, cationic polyacrylamide (CPAM) and cationic wet strengthresin PAE can all improve sizing performance in varying degrees and theamphoteric starch is more effective. Furthermore, the coordination of amphotericstarch and CPAM can promote sizing performance. Stock pH is a major factor inrosin sizing. When sized with PAC, available sizing degree can be achieved untilpH is up to 7.5. Fillers are harmful to sizing performance, especially groundcalcium carbonate. Sizing can also be affected by kind of pulp, pressure, dryingtemperature and so on.There is hardly difference among the PACs of different total aluminiumconcentration and basicity as the retention and drainage aids. PAC, amphotericstarch and CPAM can all improve the first pass retention (FPR) and drainage ofstock made from bleached wheat straw pulp to a certain degree. PAC can greatly