Using fluorescence correlation spectroscopy, we investigated the diffusion dynamics of a molecular probe in solutions of polystyrene and polybutadiene with different molar masses M-W and molecular architectures (stars, combs) over a broad concentration range (c). Whereas the tracer diffusion coefficient D was found to be insensitive to variations in M-w and architecture, the obtained master plot D(c) was polymer-specific, reflecting additional friction effects related to the concentration-dependent glass transition temperature T-g(c). This effect is fully suppressed for the same probe diffusion in poly(dimethylsiloxane) solutions with very low and virtually concentration-independent T-g. Hence, a universal master curve of D vs c can be obtained when T-g(c) is accounted for.