Tracer diffusion coefficients of camphorquinone in polypropylene glycol (PPG) with molecular weight 400, 1000, 2000, and 4000 have been measured as a function of temperature by using the forced Rayleigh scattering technique. At a fixed temperature, the diffusion coefficient decreases with increasing molecular weight of PPG. A master curve is obtained when the diffusion data are normalized with respect to the viscosity of the PPG liquid. A modified Stokes-Einstein equation for tracer diffusion is used to calculate the effective hydrodynamic radius R-*. Below room temperature, R-* is found to decrease with decreasing temperature. The decrease of R-* is discussed in terms of dynamic heterogeneity and rotation-translation coupling mechanisms. The rotation-translation coupling mechanism has recently been proposed to account for the enhancement of the tracer diffusion coefficient in the molecular liquid in the supercooled state. Comparing R-* calculated for CQ in PPG with that for the same tracer in salol in the supercooled state over the same temperature range, one finds that R-* in PPG decreases consistently less rapidly than in salol. The result is consistent with the fact that the rotation-translation coupling in PPG is considerably weaker than in liquid salol. (C) 2003 American Institute of Physics.