Self-diffusion coefficients of 1,3,5-triisopropylbenzene (TIB) in binary solutions with poly styrene and ternary mixtures comprised of TIB, toluene, and polystyrene have been measured by static gradient nuclear magnetic resonance (SG-NMR) techniques. These data, as well as mutual-diffusion coefficient data measured by capillary column inverse gas chromatography (CCIGC), have been analyzed here with the Vrentas-Duda free-volume diffusion model. Although both binary and ternary diffusion coefficient data can be accurately correlated with the model, the results contradict those of an earlier investigation, which suggested that TIE diffuses as a single unit in polystyrene solutions. The new data suggest that either : 1) TIE diffuses by piece-wise displacements in polystyrene solutions, or 2) a diffusive activation energy ceiling value has been reached. The implications of both possibilities are discussed in the context of established free-volume principles.