We present a non-standard, straightforward procedure for a model-independent determination of crucial physical parameters of the linear section of stellar jets, such as the hydrogen ionization fraction x and the temperature T e of the emitting gas. The method can be easily applied to every Herbig-Haro jet for which the brightest red lines have been measured, even if the lines are not calibrated and the ratios are not corrected for reddening. In the cases of HH 34 and HH 111 we find x∼0.1 and T e∼6000. The momentum rates evaluated with the derived total number densities (N∼104 cm−3) give strong support to the picture in which the luminous jet is only the fastest and most ionized component of a wider neutral flow that, in turn, has the capability of accelerating a molecular outflow. The ionization fraction cannot be explained in terms of equilibrium processes reflecting the local physical conditions of the medium, however the recombination length scales turn out to be comparable to the linear extension of the most luminous part of the jets: the observed degree of ionization is most likely the remnant of the heating and the excitation occurred in the initial accelerating region. This picture is supported by our diagnostics of RW Aur’s optical jet, for which spatially resolved spectra have been taken: in this case the ionization fraction (around 10% again) is found to decrease slowly with distance from the star.