The chain segmental density and diffusional properties of arborescent (highly branched) polystyrenes were investigated by labeling the polymers with pyrene and observing their quenching behavior. A series of arborescent polymers with short (5000 g.mol(-1)) branches and a linear polymer were investigated. Nitrobenzene and nitrated linear polystyrene were used as quenchers for the pyrene fluorescence. For the nitrobenzene quenching experiments, quencher diffusion coefficients were calculated using the Smoluchowski equation. Comparison of the diffusion coefficients indicates a reduced diffusion rate for small molecules inside arborescent polystyrenes relative to the linear polymer and a decreasing trend with increasing generation number. Quenching experiments with linear nitrated polystyrene led to downward curvature in the Stern-Volmer plots, suggesting that a portion of the chromophores were not accessible to the quencher groups. The fractional Stern-Volmer model was applied, and the quenching rate constants and fraction of accessible chromophores were obtained. The arborescent samples had reduced quenching rate constants, and the fraction of accessible chromophores decreased for higher generation polymers.