Monte Carlo NPT simulations were performed for water at room and supercritical thermodynamic conditions using the TIP5P model potential. The topology of the hydrogen bond patterns was characterized by local (clustering coefficient, path length and degree distribution) and global (spectral analysis) properties. The analysis was performed on 500 uncorrelated configurations at room and supercritical (500 bar and 673 K) conditions. Small-world behavior (highly clustered and small path lengths) was characterized for supercritical water, whereas for room condition, a giant cluster appears and percolates the whole system. These results might have important consequences upon using (near-) supercritical water as new reaction media. (C) 2004 Elsevier B.V. All rights reserved.