Plasticizers modify the mechanical properties of polymeric materials. The effects of plasticizers on glass transition temperatures can be most clearly observed in isochronal temperature sweep profiles of viscoelastic dynamic moduli. However, no simple mathematical models of plasticization are available to those who wish to design and employ plasticized materials in specific applications. We extend a phenomenological, molecular-level model (known as the tube-junction model) for crosslinked polymers to describe the effect of plasticizers on dynamic moduli. We show that the increase in free volume fraction due to the presence of the plasticizer can account for the shift in the glass transition in dynamic moduli. We also show that the secondary effects of plasticizers on the shape of the temperature sweep profiles can be explained in terms of increased width of the distribution of activation energies associated with intermolecular frictional forces. (C) 2000 The Society of Rheology. [S0148-6055(00)00702-1].