As2O3 glass was prepared by heating As2O3 in the evacuated, sealed silica-glass tube under the conditions given in Table 1, followed by quenching in water. Binary Na2O-As2O3 glasses containing less than 50mol% Na2O were prepared by melting the batches composed of As2O3 and NaAsO2 in the evacuated, sealed silica-glass tubes for 15-30min at 700°C with subsequent quenching in water or acetone-dry ice mixture. Density, refractive index, thermal expansion, crystallization behavior and infrared absorption of the glass were measured.The expansion coefficient of As2O3 glass was measured to be 37.3×10-6/deg, which was much larger than that of B2O3 or P2O5 glass having a layer structure, and the infrared absorption peaks of As2O3 glass in a nujol mull were found at the positions entirely identical with those of arsenolite in a KBr pellet. The absorption peak at 600cm-1 found for the KBr pellet was probably due to some mechanochemical changes of arsenolite which is considered to take place during pressing for pelletizing KBr. It was, therefore, inferred that As2O3 glass comprised a majority of As4O6 molecules and a small amount of layer- or chain-like linkage of AsO3/2 pyramids distributed over at random.In the composition region of low Na2O content in the binary glass system, the glass showed rapid decreases in thermal expansion coefficient and molar volume with increase of Na2O, and in its infrared spectrum a shoulder was found at about 830cm-1. Moreover, when the glass was heated, the crystal of claudetite II as a metastable phase of the system was separated out of it. These revealed that the glasses changed the structure from that of As2O3 glass rich in As4O6 molecules into a layer network of AsO3/2 pyramids with one or two nonbridging oxygens per each pyramid. The break of molar refractivity-composition curve at about 15mol% Na2O was presumably due to the disappearance of As4O6 molecule in the network. The experimental facts that the crystal of NaAsO2 was separated out of the glasses containing more than 20mol% Na2O and the infrared spectra of the glasses with 40-50mol% Na2O were similar to that for crystalline NaAsO2, revealed the resemblance of the structure of glasses in the region of high alkali content to that of crystalline NaAsO2.It was assumed that As3+ ion was impossible to have tetrahedral coordination because of electrostatic repulsion between the nonbridging oxygen and the unshared electron pair occupying one of sp3 hybrid orbitals of As. This assumption was supported experimentally by the fact that Tg and Mg of the glasses in the composition region of low alkali content were considerably lower than those of As2O3 glass.