A high-frequency crossed-beam correlation (CBC) experiment was performed to determine the mean-squared fluctuating density, convection speed, and characteristic turbulent coherence length of a supersonic turbulent mixing layer. Aero-optical conditions were representative of actual flight. Orthogonal helium-neon laser beams intersected to interrogate a 100 micrometer -- diameter volume. Beam motion was sensed by two quadrant detectors, whose output signals were recorded after being digitally sampled at a 5 MHz rate. Cross-correlation of angular beam deviations was computed, and from this, the mean squared fluctuating density was determined. By offsetting the beams in the streamwise direction, convection speeds were determined, enabling turbulent cell sizes to be estimated. RMS densities reached approximately 15% of the local mean density in the mixing layer, and correlation length estimates ranged from 1.5 to 2 mm. Fluctuating densities were lower, and correlation lengths were higher than predicted by a simple model. This paper summarizes experimental design and procedures, and provides a theoretical treatment of the results.