Hybrid silica spheres (HS) of size 270−350 nm with vinyl and aminopropyl surface groups were incorporated in polystyrene (PS), and its eﬀect on dielectric properties, coeﬃcient of thermal expansion (CTE), and strength of PS−HS composite was studied. Incorporation of HS in PS followed a decrease in the dielectric constant from 3.2 for PS to 2.6 for composite with 7.5 vol % HS. The decrease in the dielectric constant was attributed to (i) increased interfacial porosity, (ii) formation of anhydrous HS having low dielectric constant, during hot processing of the composites, and (iii) dispersion and preservation of the anhydrous HS in the hydrophobic matrix. The dielectric constant of the composites with HS content up to 7.5 vol % does not vary much with temperature in the range from −20 to 65 °C. These composites also exhibited reduced CTE and improved ﬂexural strength/stiﬀness due to good interfacial bonding through HS vinyl groups and dispersion of the ﬁller in the matrix. The dielectric loss increased with HS content, and the loss measured for 7.5 vol % PS−HS composite was 6 × 10−3, as compared to 10−4 for PS. At HS loading above 7.5 vol %, the tendency of HS to agglomerate and form percolated structure lead to an increase in the dielectric constant and decrease in the mechanical properties of the composites (Langmuir 2012, 28, 9742-9747).