The microwave-assisted protocol for hydrothermal fabrication of alpha-MoO3 nanobelts is suggested, Phase composition, morphology and microstructure of the samples were characterized by XRD, SEM methods and N-2 adsorption/desorption isothermal analysis. In comparison to the alpha-MoO3 nanobelts obtained after conventional autoclave method, the new nanoparticles possess both the smaller diameters and the smaller lengths. Electrochemical performance of the MoO3 electrode material for determination of alkali-metal cations is driven by specific surface area defined via nanomaterials' effective porosity. The difference in the detection limit among alkali-metal cations is explained by thermodynamic analysis of experimental and quantum-chemical data on the reduction within MoO3 lattice.