  We consider a Navier-Stokes-Fick-Onsager-Fourier system of PDEs describing
mass, energy and momentum balance in a Newtonian fluid with composite molecular
structure. For the resulting parabolic-hyperbolic system, we introduce the
notion of optimal regularity of mixed type, and we prove the short-time
existence of strong solutions for a typical initial boundary-value-problem. By
means of a partial maximum principle, we moreover show that such a solution
cannot degenerate in finite time due to blow-up or vanishing of the temperature
or the partial mass densities. This second result is however only valid under
certain growth conditions on the phenomenological coefficients. In order to
obtain some illustration of the theory, we set up a special constitutive model
for volume-additive mixtures.
