Oxidation behavior of R(4)Si(2)O(7)N(2) (J-phase; R = Y and rare-earth element) has been investigated by thermometric measurements in air. All R(4)Si(2)O(7)N(2), of which the powder samples are prepared by gas-pressured sintering method under 1 MPa of N(2) gas, are oxidized to R(2)SiO(5) with an exothermic reaction in air under an ambient pressure. The oxidation temperature to produce the final oxide increases from c.a. 930 to 1060 degrees C with decreasing the ionic radii of the rare earth elements.