The effects of three tackifiers on the glass transition temperature, terminal relaxation time, plateau modulus, and steady shear viscosity of polyolefin-based pressure-sensitive adhesives (PSAs) were investigated. Free volume theory and the Gordon-Taylor equation are used to explain the special effects of tackifiers on the glass transition temperature of the PSA systems. The plateau modulus and zero shear, viscosities were determined from which entanglements and monomeric friction. coefficients were calculated. The terminal relaxation time (related to the whole molecular chain relaxations) was calculated from the plateau modulus and zero shear viscosity. Explanations were offered as to why tackifiers have "paradoxical" effects on the viscoelastic properties of the polyolefin-based PSA, such as increasing the glass transition temperature but decreasing the plateau modulus of the base polymer. (c) 2005 Wiley Periodicals, Inc.