Experimental measurements of transitional separation, caused by a compression corner on a two-dimensional model were analyzed in an attempt to correlate the extent of separation. A previously published correlation, based on the wetted length Reynolds number at the separation point, failed when the plate length alone was changed. A re-examination of the data showed that the true correlating parameter was the freestream unit Reynolds number. Examination of the equation of motion applicable to the dividing streamline between the boundary layer and the recirculating flow suggested that the unit Reynolds number determined the growth of turbulence in the transition region. The separated flow transition could then be related to attached-flow , boundary-layer transition on a plate, with the same external flow condition. A new correlation showed that the length of transitional separation was a function only of freestream Mach number, inviscid flow pressure rise at the compression corner, and the length over which transition develops in an attached-flow, flat-plate boundary layer with the same external flow conditions.