The mechanism of retention of alkenyl succinic anhydride (ASA), which is one of the neutral sizes for papersheet, on pulp fibers at wet-end of papermaking process was studied in terms of roles of carboxyl groups in pulp fibers and those of cationic polymers and aluminum sulfate (alum) added to pulp suspension. ASA content in handsheets were determined using pyrolysis-gas chromatography (PY-GC). The results obtained using carboxylgroup-blocked pulp showed that carboxyl groups present in pulp fibers had a role to form ionic linkages with cationic ASA emulsion particles, and thus the ASA retention was mostly governed by the carboxyl groups of pulp fibers. The additions of cationic polymer and alum to pulp suspension resulted in increases in ASA retention on handsheets, and consequently their sizing features were improved. Some ASA emulsion particles originally having cationic surface charges are turned to those having amphoteric surface charges by alkenyl succinic acid (ASAcid), which are formed from ASA by hydrolysis on the surfaces of the ASA emulsion particles. Therefore, further retention of ASA may have been brought about on papersheet by additional ionic linkages formed between anionic pulp fibers, amphoteric ASA emulsion particles, cationic polymers and cationic aluminum compounds.