Oxythiophosphate compounds, which contain anions comprised of oxidized phosphorus bound to oxygen and sulfur, are scarce and, in general, poorly characterized. Although alpha-Na3PO3S has been known for over 60 years, little has been revealed about this compound. Here we present thermal analysis and second-harmonic-generat on (SHG) efficiency of alpha-Na3PO3S. Details about its transformation to the high-temperature beta phase are discussed. Under vacuum, o.-Na3PO3S undergoes one endothermic event at 553 degrees C upon heating and two exothermic events at 504 and 467 degrees C upon cooling. beta-Na3PO3S can be trapped upon quenching Na3PO3S from high temperature. We have observed that the phase converts back to the alpha phase at room temperature. Interestingly, relative humidity has been determined to catalyze this phase transformation. alpha-Na3PO3S oxidizes at 315 degrees C in air to produce Na4P2O7 and Na2SO4. Upon exposure to 1064-nm incident radiation, noncentrosymmetric alpha-Na3PO3S produces an SHG efficiency 200 times that of alpha-quartz and is nonphase-matchable (type 1).