Although many polymer nanocomposites have been described, the impact of colloidal nanoparticles (NPs) used as fillers on the thermal behavior of the matrix has been described only in a few cases. Here we apply thermal methods to inquire the effect of organically capped Ag and Au particles on poly(tert-butyl acry- late) (PtBA) based composites. The nanocomposites have been prepared by ex situ and in situ methods; in the former method the metal NPs were isolated from the respective colloids and then blended into cast films obtained from tetrahydrofuran solutions of the polymer; while in the latter the in situ poly- merization of tert-butyl acrylate using miniemulsions was carried out in the presence of homogeneously dispersed colloidal NPs. The nanocomposite materials were then characterized by differential scanning calorimetry (DSC) to assess the effect of colloidal metal NPs on the glass transition temperature of the polymer (Tg ) namely by considering their chemical nature, average particle size, and metal load. Although similar effects have been observed when Au or Ag NPs were employed as nanofillers, it was also found that the Tg depends on the NPs morphological characteristics and on the preparative method employed to obtain the nanocomposite. This research also highlights the relevance of the surface chemistry of colloidal nanofillers on the thermal behavior of the respective composites.