The structure of aggregates formed from poly(vinyl butyral) (PVB) in tetrahydrofuran (THF) was studied by transmission electron microscopy (TEM). We have found that the primary associated particles are nearly spherical and, as the association advances, the particles lengthen. Eventually aggregates branch to form a three-dimensional network. The bulk PVB was investigated by scanning electron microscopy (SEM). We have found that the bulk PVB grains are aggregates of the particles the shape and dimension of which are similar to those of the primary associated particles formed in PVB solution.