The refractive index of water was measured at atmospheric pressure under magnetic fields up to 10 T and found to increase by similar to0.1% with increasing magnetic field strength. In contrast, the refractive index of saturated aqueous electrolyte solutions decreased under the magnetic field. In the case of n-hexane, any change of the optical property by the magnetic field was not found. A possible explanation is that the lifetime of the hydrogen bond is prolonged due to the electron delocalization of a water dimer under the magnetic field.