Coherent anti-Stokes Raman spectroscopy (CARS) of the symmetric CH and CC stretching vibrations of acetylene has been used to follow the formation and cooling of nanoclusters formed in free jet expansions, The internal temperatures and phases of the clusters have been determined by a comparison with the spectra obtained for equilibrium samples at low temperatures. For expansions of neat acetylene, the first-formed cluster was supercooled liquid at 176 K, a somewhat surprising result since the liquid phase does not normally exist at pressures of 1 atm or less. For a 10% C2H2/Ne expansion, no liquid phase of clusters was observed and the nascent cluster was cubic solid at 135 K. Both results are shown to be consistent with a simple model of homogenous condensation along the expansion isentrope. Studies of neat and mixture expansions show gradual and definite phase transitions from liquid to cubic solid to orthorhombic solid as the cluster cools along the flow axis of the expansion. Evidence that the solid clusters are crystalline rather than amorphous is seen in the appearance of resolved factor group components in both cubic and orthorhombic phases. Although the freezing process is fast (similar to 0.3 mu s), the conversion from cubic to the orthorhombic form appears less facile, requiring similar to 4 mu s. If one assumes that the rate is limited by evaporative removal of the transition enthalpy, a mean cluster diameter of 140 nm is deduced.