We investigated the structures of polyelectrolyte gels, poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropane sulfonic acid) (NIPA/AMPS) hydrogels in a unidirectionally swollen state by using small-angle X-ray scattering (SAXS). The SAXS results show that the structure of the NIPA/AMPS gels strongly depends upon the composition of NIPA/AMPS. Increase in composition of AMPS causes suppression of concentration fluctuations in the long wavelength. As a consequence, a NIPA/AMPS hydrogel with a low composition of AMPS macroscopically phase-separated at high temperatures, while microphase separation occurred for a NIPA/AMPS gel with a higher composition of AMPS. The instability in the microphase separation initially occurred in the direction perpendicular to the swelling for the latter gel. In the disordered state near the microphase separation region, an elliptic scattering pattern was observed, and the scattering intensity around the peak position in the direction perpendicular to the unidirectionally swelling was larger than that in the direction parallel to it. The behavior became more remarkable, as the interaction parameter chi became larger. These behaviors are consistent with the prediction from the Rabin-Panukov theory. The scattering vector at the scattering maximum in the perpendicular direction qm, significantly shifted to smaller q, where q represents the magnitude of the scattering vector, when the microphase separation occurred. It is shown that the periodicity of the microphase-separated structure ranged from 300 to 400 angstrom. (C) 2011 Elsevier Ltd. All rights reserved.