A series of inorganic-organic molecular hybrids, poly(styrene-co-octavinyl-polyhedral oligomeric silsesquioxane)s (PS-POSSs), were synthesized, and their structures and properties were characterized by FTIR, H-1-NMR, Si-29-NMR, XRD, optical microscopy (OM), and atomic force microscopy (AFM). The chemical incorporation of POSS into polymer matrixes achieves uniform dispersion and makes the resultant hybrids display good film formability. The relationship between molecular structure of these hybrids and their dielectric constants and formation mechanism of low dielectric constant were investigated. The low dielectric constant of the hybrids mainly originates from the increase of free volume, involving the free volume of intrinsic porosity from POSS and an increase in the free volume owing to steric hindrance of bulky POSS. The latter plays a dominant role to increase the free volume and formation of low dielectric constant. Simultaneously, the polymer arm-length has an important influence on the dielectric constant of the star-type hybrids. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 2628-2634, 2013