The mechanism of the denaturing effects of urea and the guanidinium ion on proteins is still an unsolved and important problem in protein chemistry. Changes in the hydrogen bond network of water in the first hydration shell of urea and guanidinium were analyzed in terms of the random network model using Monte Carlo simulations. Bulk water consists of two populations of hydrogen bonds : a predominantly linear population and a small but significant population of slightly longer and more bent hydrogen bonds. In the first shell of urea, hydrogen bonds between waters solvating the amino groups were shorter and more linear on average than those in bulk water. These changes are caused by a depletion of the more distorted hydrogen bonds. These changes in hydration water structure have previously been seen only around nonpolar solutes of solute groups. Thus urea, being entirely polar, is anomalous in this regard. Hydrogen bonds around guanidinium were longer and more bent than those in bulk water. These distortions are characteristic of a polar solute but are smaller than expected for an ion. The hydrogen bond structural parameters were combined with a random network model equation of state for beat capacity to calculate the hydration heat capacities (Delta C-p) of urea and guanidinium. The value of Delta C-p obtained for urea is positive, characteristic of a nonpolar solute, and in good agreement with the experimental value. Urea and, to a lesser extent, guanidinium are unique among polar molecules in that they are highly soluble yet appear to structure water more like nonpolar solutes. The relevance of this observation to proposed mechanisms of denaturation is discussed.