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In order to elucidate the mechanism of enhancement of heat transfer in polymer composites, in this work, we investigated two types of polymer-carbon filler composites. This investigation was made using scanning thermal microscopy (SThM) with the Wollaston microprobe operated in active mode as a function of the carbon filler weight fraction within the polymer matrix. Samples consist of high-density polyethylene (HDPE) filled with 50 µm expanded graphite (EG) and polyvinylidene difluoride (PVDF) containing multiwall carbon nanotubes (MWCNTs). For HDPE/EG samples, SThM images allow the detection of zones with a thermal conductance larger than that of the matrix for the highest studied filler concentration. These zones correspond to EG filler agglomerations within the polymer and explain the observed enhancement of the thermal conductivity k of the HDPE/EG composite. For PVDF/MWCNTs samples, it is found that k increases from 0.25 W m−1 K−1 for pristine PVDF to 0.37 W m−1 K−1 for PVDF nanocomposites filled with 8 wt. % MWCNTs. This k variation vs filler concentration is found in good correspondence with that of the β phase relative percentage in the PVDF nanocomposites. This suggests that the observed heat transfer enhancement is rather due to the formation of β phase for PVDF/MWCNTs samples, resulting from the addition of MWCNTs than the addition of MWCNTs itself. Thus, tuning the thermophysical properties of polymer-based nanocomposites can establish new design laws to confer them specific thermal properties. [ABSTRACT FROM AUTHOR]