Modification of biocompatible elastomers, especially polydimethylsiloxane (PDMS), through blending with hydrophilic excipients is sought in various biomedical applications to enhance the affinity of the hydrophobic polymer to water and increase the permeability of water vapor and of bioactive substances. In this work, a rubbery poly(ethylene oxide)/ poly(propylene oxide)/poly(ethylene oxide) block copolymer, known as poloxamer 188 (POL), and glassy polyvinylpyrrolidone (PVP) were incorporated in PDMS matrices at a 2.5-35% w/w content. POL, due to its amphiphilic nature, was better dispersed in smaller spherical domains than PVP in the matrix and exerted a stronger interfering effect on the curing reaction of PDMS, as indicated by DSC data. The Young modulus of PDMS-POL films decreased with increasing POL content and that of PDMS-PVP films remained practically unaffected up to a 20% w/w PVP content and exhibited an abrupt increase above 25% w/w. For a certain additive content, the water uptake was higher in the PDMS-POL films and produced a controllable and gradual increase in the permeability of caffeine over the whole range of additive content studied. Above a 25% w/w content, the PDMS-PVP films are characterized by a rather abrupt increase in caffeine permeability values, well above the maximum increase predicted by Maxwell's model for strictly two-phase systems with fully discrete phase domains of the dispersed phase. The effects of PVP and POL on the examined properties of the modified PDMS matrices are discussed and interpreted in relation to the morphological features of the films with increasing additive content and the network characteristics of the PDMS phase.