We have investigated the p-Hg1-xZnxTe interface, with x in the range of 0.19-0.23, after electrochemical treatments with Na2S in ethylene glycol using admittance spectroscopy of metal-insulator-semiconductor structures made on these passivated surfaces. Native sulfide layers are characterized from angle-resolved x-ray photoelectron spectroscopy measurements and are constituted mainly by ZnS. The surface roughness is ascertained by atomic force microscopy. Free carriers densities and the degree of compensation for the semiconductor are deduced from capacitance measurements : the method is based on a numerical analysis of the normalized capacitance measured at high frequency as a function of the bias voltage. The positive fixed charge density in the native sulfide layer is found to be in the range of 10(10)-10(11) cm(-2) as deduced by the shift in the C-V characteristics. C-V plots show a very small hysteresis effect at 77 K, resulting in a low density of slow surface states (similar to 1 x 10(10) cm(-2)). The energy distribution of fast interface states is calculated in two ways from capacitance and conductance measurements. The conductance method shows a higher sensitivity. A broad U-shape distribution is obtained for the different samples studied, with a reproducible minimum density of about a few 10(10) eV(-1) cm(-2); The best passivation conditions are obtained with a constant current density of about 100 mu A cm(-2) and a sulfidization duration of similar to 15 min. The time constant of the interface traps is deduced from the energy loss peak and leads to an estimation of their capture cross section. All these results show that a very high interface quality can be obtained on p-Hg1-xZnxTe in a reproducible way by anodic sulfidization.