A series of closely related polyphosphazenes with propoxy, pentoxy, hexoxy, octoxy, isostearyloxy, and 2-(2-methoxyethoxy)ethoxy (MEE) side groups, together with cosubstituent species with both the alkoxy and MEE side chains, were studied for their morphology and miscibility with oligoisobutylene (OIB). All the pure polymers except one had a single glass transition temperature. The exception was the species with both isostearyloxy and MEE side groups, which underwent two low-temperature second-order transitions, even though 31P NMR spectra indicated the absence of a blocktype structure. For the single-substituent macromolecules, the solubility at 80 degrees C in OIB increased as the length of the unbranched alkoxy side groups rose from propoxy to octoxy (from 1 to 11 wt/wt%). However, the polymer with two isostearyloxy side chains per repeat unit had a low solubility in OIB (3 wt/wt%), and the species with the two MEE side groups on every repeat unit was totally insoluble. When both alkoxy and MEE side groups were present, the solubility in OIB was also low (0-3%), except for the species with both isostearyloxy and MEE side groups, which was soluble in OIB at a level of 21 wt/wt% at 80 degrees C, and showed T(g) evidence of polymer/oligomer miscibility even at -80 degrees C. Explanations are suggested for the unusual behavior of this polymer. POLYM. ENG. SCI., 51:1693-1700, 2011. (C) 2011 Society of Plastics Engineers