Results of speed-of-sound measurements in a (0.49896 argon + 0.50104 carbon dioxide) mixture. Relative deviations of experimental speeds of sound cexp from values cEOS calculated with the EOS-CG equation of state of Gernert and Span [1] are plotted versus pressure p. ◇, non-radial mode (1, 1) (including error bars for (0.042 to 0.055)% measurement uncertainty, k = 2); △, radial mode (0, 3); □, radial mode (0, 4). Symbols plotted in red indicate points that may have been affected by pre-condensation.