Development of sizing with rosin and alumina is explained in terms of surface-chemical parameters. Premature agglomeration, inadequate orientation, inadequate amount of alumina, and formation of unbound rosin soap at high pH are the key factors that, to varying degrees, affect the sizing development of both forms of rosin. Common mill problems and practices of optimizing sizing are discussed in the context of the chemical differences of soap-type and dispersion-type rosin sizes. -- AATA