An interferometric arrangement that automatically compensates for thermal expansion was used to examine the changes in the refractive index of liquid water at 632.8 nm as a function of temperature from 10 to -15 degrees C. By combining the results of this research with existing data, we calculated the absolute refractive index in the supercooled region with an accuracy ranging from 3 x 10(-6) to 1 x 10(-6). A direct observation of the refractive-index maximum of water is reported for the first time to our knowledge and found to occur between 0 and 0.1 degrees C.