Small-angle neutron scattering (SANS) experiments were performed on mixtures of 3-methylpyridine, D2O, and salts to investigate concentration fluctuations and structures in the vicinity of the critical point. The temperature-dependence of the SANS profile of a mixture without salt showed critical phenomena that were consistent with 3D-Ising behavior. On the other hand, the SANS profiles with salt added mixtures could not be explained solely in terms of critical scattering, because they include both the concentration fluctuation and a periodic structure. The periodic structure could be induced by the coupling of solvation and a critical concentration fluctuation. (c) 2006 Elsevier B.V. All rights reserved.