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This study is seeking a better understanding of polyethylene glycol (PEG) as a solvent to promote its use in chemical synthesis. The effect of adding two solutes of interest, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) and 5-tert-butylisophthalic acid (5-TBIPA) to PEG200 (average molar weight of 200 g·mol−1) on the solution density, viscosity, and selfdiffusion coefficients is monitored in a temperature range of 298.15–358.15 K to deduce how these solutes interact with the PEG200 solvent. The effect of water, the most common impurity in PEGs, is also monitored and found to be nearly negligibly small. Addition of (5-TBIPA) increases solution density and viscosity. Combined with the observation that 5-TBIPA consistently self-diffuses at about half the rate as PEG200 at all investigated experimental conditions, this suggests strong attractive solute–solvent interactions likely through hydrogen bonding interactions. In contrast, addition of TEMPO causes lower solution densities and viscosities suggesting that the solute–solvent interactions of TEMPO lead to an overall weakening of the intermolecular interactions present compared to neat PEG200. Inspection of the viscosity and self-diffusion temperature dependence reveals slight deviations from the Arrhenius equation. Interestingly, the activation energies obtained from the viscosity and the self-diffusion data are essentially identical in values suggesting that the same dynamic processes and thus the same activation barriers govern translational motion and momentum transfer in these PEG200 solutions.