Fluorescence correlation spectroscopy (FCS) was employed to study the diffusion of molecular tracers in different polymer melts (polydimethysiloxane (PDMS), 1,4-cis-polyisoprene (PI), poly(vinylethylene) (PVE), and a symmetric PI/PVE blend) as a function of molecular weight (M-w) and temperature (7). The single molecule sensitivity of the FCS technique precludes any modification of the matrix polymer properties. In all studied systems, the small tracer diffusion coefficient D(M-w,T) senses local segmental dynamics depending oil the glass transition temperature T-g(M-w) of the polymer matrix and not its macroscopic viscosity. From the good representation of the D(7) data by the common non-Arrhenius (VFT) function, we found that the activation energy (B-D) increases with tracer size (R) and for I given tracer the value of B-D in PI is almost 2 times bigger than in PDMS. The possibility to establish a direct relation between D(T) and the segmental relaxation time tau(T) of the polymer matrix was critically addressed based on experimental data in dynamically homogenous (homopolymers) and heterogeneous (miscible blend) systems and discussed in view of recent computer simulations of polymer/penctrant mixtures.