The present invention provides a composition comprising (a) a thermally decomposable polymer and (b) a siloxane oligomer evenly dissolved in (c) an organic solvent; a composition comprising (a) a thermally decomposable polymer, (b) a siloxane oligomer, and (c) an organic solvent in which both of the ingredients (a) and (b) are soluble; a method for forming a low-permittivity film characterized by applying the composition to a substrate to form a composite film comprising the thermally decomposable polymer and the siloxane oligomer evenly compatibilized therewith and then heating the resulting film to condense the siloxane oligomer and remove the thermally decomposable polymer; a method for forming a low-permittivity film characterized by applying the composition to a substrate to form a composite film comprising the thermally decomposable polymer and the siloxane oligomer evenly compatibilized therewith, subsequently conducting a first heating step in which the siloxane oligomer is crosslinked while keeping the thermally decomposable polymer remaining in the film, and then conducting a second heating step in which the thermally decomposable polymer is removed; a low-permittivity film formed by either of the methods for low-permittivity film formation; and an electronic part having the low-permittivity film.