Surface structure and relaxation phenomena have been investigated for thermally poled 20WO(3) (.) 80TeO(2) glass, which is known to show a large second-order nonlinear susceptibility. X-ray photoelectron spectroscopy reveals that the penetration of Na+ into the anode-side surface of 20WO(3) (.) 80TeO(2) glass occurred during poling, since the poling was performed with the 20WO(3) (.) 80TeO(2) glass sandwiched in between two commercial borosilicate glasses containing Na+. The agreement between dependence of second-harmonic intensity on etched thickness at anode-side surface and the concentration profile of Na+ suggests that the penetration of Na+ predominantly contributes to the second-harmonic generation. The decay of second-harmonic intensity at room temperature is describable in terms of the single exponential function except for the glass poled at higher temperature, for which the stretched exponential function is applicable. The activation energy for the decay of second-harmonic intensity of the glass poled at 260 degreesC is 47 kJ (.) mol (1). This value presumably corresponds to the diffusion of Na+ ions. (C) 2002 Optical Society of America.