Fourier Transform Infrared Spectroscopy has been used to study homogeneously nucleated hydrazine (N2H4) aerosols formed in a variable temperature flow cell. The spectra contain both resonant absorptions and Mie scattering features which allow detailed information to be obtained on the phase of the aerosol particles and the size distribution that is formed. In addition, the relative intensities of the absorption and scattering features can be used as a qualitative probe of the density of the particles. A disordered phase is observed at cell temperatures ranging from 230 to 180 K and a transition to a crystalline phase occurs over the temperature range 180--175 K. At lower temperatures the infrared spectra become progressively narrower, indicating that the quality of the crystals improves as the temperature is lowered. The result is previously unobserved details in the hydrazine N-H stretch vibrations of the crystalline solid. The high quality of the spectra obtained under these conditions suggests that at the high condensation rates characteristic of the present experiments, self-annealing yields highly crystalline particles. Mie calculations, based upon the complex refractive index data for amorphous solid hydrazine, are shown to have utility in modeling the infrared spectrum of the disordered phase, which we tentatively assign as a supercooled liquid. ¿ American Geophysical Union 1993