The dynamic mechanical properties of poly(n-butyl methacrylate) (PBMA) latex films postadded with alkali-soluble resin (ASR) have been studied and compared with those of latex films prepared by emulsion polymerization in the presence of ASR (ASR-fortified latex). The miscibility between PBMA and ASR, poly(styrene/alpha-methylstyrene/acrylic acid) (SAA), was found to influence the dynamic mechanical behavior of the films. The dynamic properties of PBMA latex films postadded with SAA show two distinct damping peaks, which correspond to those of PBMA and SAA, respectively, in the phase-separated state. The SAA migrates onto film surface during film formation and, as a result SAA preserved their domains in the matrix phase, showing two distinct relaxations in the dynamic mechanical spectrum. On the other hand, the ASR-fortified films exhibit single damping peak. SAA-fortified latex particles would be core/shell structured, and the miscibility between PBMA and SAA is clearly improved by the grafting reaction between PBMA and SAA.