Structure formation by coupling between formation of crosslinking points and liquid-liquid phase separation was investigated for aqueous methyl cellulose solution by small-angle X-ray scattering (SAXS) and light scattering (LS) techniques. The sol-gel phase diagram and the SAXS results suggested that the liquid-liquid phase separation occurred before gelation. By LS measurements, the structure due to the liquid-liquid phase separation was directly observed. By applying speckle analysis on the LS profiles, it was suggested that the gelation and the phase separation strongly coupled each other: the increase in the apparent molecular weight by crosslinking induced the liquid-liquid phase separation. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 168-174, 2010