The sonic properties of mono-component single-phase non-ideal flows are studied assuming steady, one-dimensional, inviscid, shock-free adiabatic flow. Under these hypotheses, the total enthalpy \(h^{\,\text {t}}\) and entropy s are uniform in the flow, and the sonic properties density \(\rho ^*\) , speed of sound \(c^*\) , momentum density \(\rho ^*c^*\) , and momentum flux density \(\rho ^*c^{*2}\) are readily computed from the total thermodynamic state specified by the total pressure and temperature \((P^{\,\text {t}}, T^{\,\text {t}})\) . Application-relevant fluids and operating conditions are considered and isentropic expansions from reservoir to sonic conditions are investigated using both state-of-the-art thermodynamic models and the ideal-gas model. The evolution of selected thermodynamic properties along isentropic expansions is monitored and maps reporting the sonic properties in the \(P^{\,\text {t}}\) - \(T^{\,\text {t}}\) thermodynamic plane are constructed to illustrate the mechanism of dependence of the sonic properties on the total thermodynamic states. Several different non-ideal thermodynamic effects are identified and commented, which determine remarkable qualitative and quantitative differences between ideal and non-ideal flows.