The structure and structural transformations of the water/ POE-1000 eutectic system were studied in the interval -100° to 0°C by means of light microscopy. It was established that microphase separation and eutectic crystallization takes place, resulting in the formation (along cracks initially present in the sample) of rows (bands) of crystals of the polymer-rich component, surrounded by zones of water. A spinodal mechanism is suggested for the liquid-liquid phase separation of the water/POE-1000 eutectic system when it is supercooled with a temperature gradient and then heated. A good correlation was found between the temperatures of the phase transformations (melting and crystallization) as determined by light microscopy, x-ray, and calorimetrie analyses.