Surface properties of superprotonic (K1-х(NH4)x)3H(SO4)2 (x ≥ 0.57) single crystals and their evolution under humidity were studied by optical polarization microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDXS). Traditional method of atomic force microscopy (AFM) and sophisticated methods such as Kelvin probe force microscopy (KPFM) and scanning capacitance force microscopy (SCFM) were also used. Fresh and aged cleavage surfaces perpendicular and parallel to the c axis of the trigonal superprotonic phase were examined. The effect of air humidity on the surface morphology, surface conductivity, surface potential and surface capacitance was studied. The influence of material composition and surface orientation was considered. During ageing for 430 h, the (001) sample surfaces, initially stepped, were smoothened, surface electric potential changed from negative greater than a hundred of millivolts in absolute value to positive of about 80 mV, and a modified layer containing new crystal phases was formed. •Superprotonic phases (K1-x(NH4)x)3H(SO4)2 (x ≥ 0.57) are stabilized at room temperature and demonstrate high proton conductivity.•Their surface properties were studied by AFM. C-AFM, KPFM, SCFM, optical and electron microscopy, and structural analysis.•Local CVCs have shown that conductivity is anisotropic and is due to the composition and structure of superprotonic phases.•Based on morphological data, the stability of the surface of samples of different composition to humidity was assessed.•The formation of a modified layer with new phases on the crystal surface during prolonged exposure under humidity is shown.