We have measured the dynamics of n-buoxyethanol-water mixture of alcohol mole fraction 0.09 by using a neutron spin echo (NSE) technique at 8, 25, and 37 °C, below a lower critical solution temperature (LCST) of 49 °C where n-butoxyethanol-water mixtures undergo phase separation. The diffusion coefficients obtained from intermediate correlation function were determined as 3.3×10-11, 6.6×10-11, and 8.5×10-11 m2s-1 at 8, 25, and 37 °C, respectively. These values are about 102 times smaller than those of pure water and alcohol, probably reflecting the dynamics of molecular aggregates formed in the mixture, which has been confirmed by previous small-angle neutron scattering. On the other hand, the mutual-diffusion coefficient obtained from dynamic light scattering on the same system decreases with increasing temperature, due probably to an increase in the correlation length of molecular aggregation. The time scale observed by NSE corresponds to that of formation of molecular aggregates, much shorter than that by light scattering. Therefore, it is concluded that the molecular aggregation in the mixture is transitory with a nanosecond lifetime.