The nature of fluctuation modes of lamellar structure in a nonionic surfactant/water system has been investigated using a small angle x‐ray scattering and a neutron spin echo technique. Approaching temperature from lamellar phase to double‐gyroid phase, a diffuse scattering peak appears in the small angle scattering profile prior to the transition. This diffuse scattering is originated from the least stable fluctuation modes of lamellar structure predicted by Qi and Wang. The intermediate scattering profiles of the lamellar phase can be described by an undulation fluctuation mode and a least stable fluctuation mode of the lamellar structure. © 2004 American Institute of Physics