In comparing their results with those of Mack, Wazzanetal. [Phys. Fluids27, 331 (1984)] found major differences regarding the effect of free‐stream Mach number on viscous instability. The present work shows that all past observations concerning stability behavior in the supersonic range based on Mack’s work are accurate to the level of approximation involved. More importantly, as a consequence of discussions with Mack (private communication) prompted by this dispute, this paper clarifies the fluid‐property assumptions made by him in his earlier calculations and demonstrates the sensitivity of stability calculations to the accuracy of the basic state.