The Maron theory of polymer solutions was used to examine the 30°C vapor pressure data of Tait and Livesey on solutions of polyheptene-1, polydecene-1, polydodecene-1, and polyoctadecene-1 in toluene. For polyheptene-1 and polydecene-1, excellent agreement was obtained between theory and experiment over the entire concentration range studied. However, for the other two polymers, deviations were observed between the calculated and observed vapor pressures at higher solution concentrations. These deviations were used to show the presence of crystallinity in these and to ascertain the amounts. Further, it was shown that only one crystalline form of the polymer was present in the polydodecene-1, whereas in the polyoctadecene-1 two crystalline forms were detected. These observations, confirmed by measurements with a differential thermal analyzer (DTA), indicate that for polydodecene-1 and one of the crystalline forms in polyoctadecene-1 the crystallization involves alignment of both backbone and side chains; however, the lower melting form in polyoctadecene-1 appears to have only the side chains aligned in the crystal lattice.