A good determination of the mass-luminosity relation for evolved stars on blue loops can determine the degree of excess mixing in the interiors of their main-sequence companions. In this study we determine the dynamical mass of the Cepheid binary S Muscae. This can be combined with its known luminosity and be fitted on evolutionary tracks to determine the amount of mixing in the main-sequence progenitor.Using the Goddard High Resolution Spectrograph on the Hubble Space Telescope we have measured the orbital radial velocity changes for the companion of the Cepheid S Mus. Spectra taken at minimum and maximum orbital velocities were cross-correlated. The velocity difference was measured to be 30.6 +/- 0.4 km s-1. The difference for the orbital velocities of the Cepheid for the same phases was determined to be 26.9 km s-1 +/- 0.4 km s-1. This gives a velocity ratio of 1.14 +/- 0.02, which gives us the mass ratio for the companions. Adding possible centering errors of the target in the entrance apertures the error limit would be increased to +/-0.06. The derived spectral types of the companion S Mus B range from B3 V to B5 V depending on the criterion used to determine it, with an average spectral type B3.8 V. Using the average spectral type and the main-sequence mass-spectral type relation from Andersen & Harmanec we find for S Mus B a mass of 5.2 +/- 0.2 M⊙. With the newly determined mass ratio the mass for the Cepheid S Mus A comes out to be 5.9+0.7-0.6 M⊙. Taking the mass of 5.9 M⊙ at face value and adopting the absolute visual magnitude of Mv = -4.29 (log L/L⊙ = 3.62) for the Cepheid S Mus this indicates mixing in its main-sequence progenitor slightly in excess of the one assumed for the Maeder and Meynet evolutionary tracks. The present uncertainties in mass and luminosity prevent, however, a firm conclusion about the exact degree of mixing. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA Contract No. NASS-26555.