Dielectric measurements of water mixtures of ethylene glycol oligomer (EGO) with 1-6 repeat units were carried out in the frequency range of 100 MHz-30 GHz at 25 degrees C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO-water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO-water cluster for smaller EGO-water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO-water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters.