New sodium ion sensitive metal-oxide-type sensors have been developed and tested with a direct solid state contact method. Performance was demonstrated at ambient temperature with single crystals of Na0.9Mo6O17 and Li0.33MoO3. The sodium molybdenum bronze sensor responded reproducibly and fast to changes of Na+ concentration in the range of 1-10(-4) mol dm(-3). Cross sensitivity tests to other ions such as H+ or K+ have shown that the new sodium ion sensor may be used when the concentration of other ions is an order of magnitude smaller than the Na+ concentration.