Wind tunnel tests were conducted on a family of 10 all movable 45-degree swept stabilizer models. The model panels were essentially dynamically identical except that panel stiffness levels were varied to obtain flutter within the Mach 0.8 to 1.2 speed range at a mass ratio of 25. For pitch to bending frequency ratios of 2.67 and 1.50, and a mass ratio of 25, flutter speeds remained between 16 and 19 percent higher than predicted by incompressible swept analyses using the methods of NACA TRI014. No significant transonic dip or supersonic rise in experimental values of VsubEbsubromegasubalpha was observed for the above conditions, however, a region of low stabillty was found to exist below the flutter boundary beyond Mach 1.0. At Mach 0.8 the flutter to torsion frequency ratio omegasub Eomegasubalpha was from 0.55 to 0.60 and was little affected by variations in increasing Mach number up to 1.2 at mu 25 resulted in a drop in of about 20 percent. Increasing mu above 25 causes further drops in omegasub Eomegasubalpha. For the configuration tested pitch does not become significant amplitude-wise until omegasubphiomegasub h1 drops down close to unity.