The optical aberrations induced by mixing layers of dissimilar gases are recorded and analyzed in order to characterize the spatial and temporal properties of the flow. Laser light was propagated through a mixing layer of Helium and Nitrogen gas, having velocities of 8.5 m/sec and 1.5 m/sec, respectively. The light was propagated in a direction perpendicular to the plane of the mixing layer. The mixing layer was evaluated in two experimental regimes: free turbulent mixing, where the mixing layer spreads into the surrounding air; and channel flow, where the mixing layer is confined to a rectangular channel. The optical perturbations induced by the mixing layer were recorded using a lateral shearing interferometer and a point spread function camera. Autocorrelation functions and structure functions were computed from the spatially resolved phase surfaces obtained using the shearing interferometer. For both the free and channel flows, the phase fluctuations were not wide-sense stationary. Consequently, the Strehl ratio predicted by traditional aero-optical models did not agree with experimental measurements except m regions of the flow where the Reynolds number was low. However, the phase fluctuations were locally homogeneous. A two-dimensional power law model was developed, analogous to the one-dimensional Kolmogorov model for isotropic turbulence. This model predicted a relative Strehl ratio which closely matched experiment throughout the flow. In a second series of experiments, the gas velocities were reduced to 4.5 rn/s and 1.0 rn/s for the Helium and nitrogen gas, respectively. The flow orientation was rotated such that the laser light propagated in a direction parallel to the plane of the mixing layer.