The temperature and shape of ice particles in polar mesospheric clouds (PMCs) were determined using observations near 3 μm wavelength from the Solar Occultation for Ice Experiment (SOFIE). The resulting ice temperatures are 5–20 K colder than the current SOFIE temperatures retrieved from CO2 transmission measurements. Particle shape is described using oblate spheroids, and the axial ratios determined in this study are slightly more spherical than previous results obtained using SOFIE observations assuming constant temperature (1.9 at the altitude of peak mass density compared to 2.3 previously). Using ice temperatures in an equilibrium PMC model results in ice mass densities that are much higher than observed. SOFIE observations indicate that the amount of H2O that can enter the ice phase is related to ice concentration, suggesting that ice nucleation plays an important role in determining PMC formation and variability. This result may also imply that the neglect of transient phenomena in the equilibrium PMC model may be a large source of error. SOFIE observations do not support the existence of amorphous ice particles near the summer mesopause but rather indicate that cubic ice is ubiquitous.