In this work, we explore the depolymerization of poly(dimethylsiloxane) (PDMS-100) and poly(methylphenylsiloxane) (PMPS) using dimethyl carbonate (DMC) and develop a surface functionalization method by utilizing the DMC-imparted active methoxy end groups of the partially depolymerized polysiloxanes. The efficiency of dimethyl carbonate as a reagent for organosiloxane cleavage was confirmed by means of 1H NMR spectroscopy, size-exclusion chromatography, and viscosity measurements. The reaction of fumed silica with organosiloxanes (PMPS, PDMS-50) in the presence of DMC was investigated using the ζ-potential, 29Si and 13C solid-state NMR spectroscopy, IR spectroscopy, CHN analysis, contact angle goniometry, thermogravimetric analysis, scanning and transmission electron microscopy (TEM), and rheology. It was found that the interaction of PMPS/DMC with an SiO2 surface produced hydrophobic and thermally stable moieties (up to 550 °C) with a densely packed (average 2.2 groups/nm2) alkylsiloxane network for SiO2/PMPS + DMC in comparison with SiO2/PMPS (average 1.4 groups/nm2). Surface functionalization was successfully attained at a relatively moderate temperature of 200 °C. Scanning electron microscopy data show that the average size of aggregates of PMPS/DMC-modified silica nanoparticles is smaller than that of the initial silica and silica modified with neat PMPS. TEM images reveal uniform distribution of the PMPS/DMC mixture across the SiO2 surface. Rheology studies show thixotropic behavior in industrial oil (I-40A), a fully reversible nanostructure and shorter structure recovery time for fumed silica modified in the presence of DMC.