AbstractCationic dry strength agents (DSAs) are commonly used for paper strengthening. However, before they are applied, another chemical called an anionic trash catcher (ATC) is often used to pretreat pulp to neutralize the negative dissolved and colloidal substances (DCS), increasing the effectiveness of the subsequently added cationic DSA. However, in many cases, the negativity of the fibers is also neutralized by the ATC. Under such circumstances, it remains unclear to what degree the effectiveness of the DSA will be affected. In this paper, a deinked pulp was first pre-treated with a polyamine ATC; then, the effectiveness of two subsequently-added DSAs, anionic polyacrylamide (APAM) and cationic polyacrylamide (CPAM), were compared. Results showed that when the ATC was used alone, it deteriorated paper formation, resulting in a decrease of paper strength. When the DSAs were used alone, the CPAM was effective but the APAM was not. However, when the ATC was used to pretreat the pulp, the effectiveness of the CPAM clearly decreased, while that of the APAM clearly increased. The reason for this could be that the adsorption of APAM onto fibers was promoted by the pretreatment of ATC, possibly through the formation of an ATC and APAM polyelectrolyte complex.