We determined the hydration numbers of 2-butoxyethanol (2BE) molecules in the monomeric and the micellar form using terahertz time-domain attenuated total reflection spectroscopy. The hydration number of the 2BE monomer is somewhat larger than that of the 2BE molecule in micelles due to the presence of immobilized water molecules around the hydrophobic surface of the 2BE monomer. The hydration number of the 2BE monomer decreases with increasing the temperature, whereas the hydration number of the 2BE micelle is almost unchanged up to 47 degrees C. This indicates that strongly bound hydration water molecules should exist and make a self-organized structure on the micelle surface. (c) 2009 Elsevier B.V. All rights reserved.