The change in morphology of merocyanine–Cd arachidate mixed Langmuir–Blodgett (LB) films induced by hydrothermal treatment (HTT) has been studied using a polarized light photomicroscope and an X-ray diffractometer. By applying HTT of 100% humidity at 70 °C for 60 min, the as-deposited J-band with an in-plane anisotropy is reorganized to an isotropic phase with a sharp peak, and the image, initially anisotropic in brightness, becomes isotropic and filled with round superstructures of 0.1 mm or larger in diameter with an average thickness of presumably less than three monolayers. The change in the X-ray diffraction pattern indicates that the Y-type LB lamellar structure remains intact with an enhanced degree of order of the Cd-ion alignment.