Although it is widely accepted that the local structure of liquid water has tetrahedral arrangements of molecules ordered by hydrogen bonds, the mechanism by which water molecules switch hydrogen-bonded partners remains unclear. In this mechanism, the role of nonhydrogen-bonded configurations (NHBs) between adjacent molecules is of particular importance. A molecule may switch hydrogen-bonding partners either (i) through thermally activated breaking of a hydrogen bond that creates a dangling hydrogen bond before finding a new partner or (ii) by infrequent but rapid switching events in which the NHB is a transition state. Here, we report a combination of femtosecond 2D IR spectroscopy and molecular dynamics simulations to investigate the stability of NHB species in an isotopically dilute mixture of HOD in D 2O. Measured 2D IR spectra reveal that hydrogen-bonded configurations and NHBs undergo qualitatively different relaxation dynamics, with NHBs returning to hydrogen-bonded frequencies on the time scale of water's fastest intermolecular motions. Simulations of an atomistic model for the OH vibrational spectroscopy of water yield qualitatively similar 2D IR spectra to those measured experimentally. Analysis of NHBs in simulations by quenching demonstrates that the vast majority of NHBs are in fact part of a hydrogen-bonded well of attraction and that virtually all molecules return to a hydrogen-bonding partner within 200 fs. The results from experiment and simulation demonstrate that NHBs are intrinsically unstable and that dangling hydrogen bonds are an insignificant species in liquid water.femtosecond 2D IR spectroscopy ͉ molecular dynamics ͉ liquids O n average, molecules in liquid water are tetrahedrally coordinated but appear to engage in 10% fewer hydrogen bonds than in ice. Support for this estimate comes broadly, from latent heats of melting and vaporization, from x-ray and neutron scattering, and in very detailed form from molecular dynamics (MD) simulations (1-3). The role of nonhydrogen-bonded configurations (NHBs) in water's rapidly changing structure remains uncertain, lying at the heart of differences between mixture and continuum models of water (1, 3-8). Implicitly or explicitly, the interpretation of many experiments and MD simulations conceives of NHBs as broken or dangling hydrogen bonds, stable species that interconvert with a hydrogen-bonded configuration (HB) at a rate determined by the free energy barrier separating them. But it is also possible that NHBs are intrinsically unstable species that appear transiently during natural fluctuations about a hydrogen bond or when molecules trade hydrogen-bonding partners. These two scenarios not only provide qualitatively different interpretations of water's structure and how it evolves, but also imply different pictures for how water mediates chemical and biological processes. We have distinguished between these scenarios by using a combination of femtosecond 2D IR spectroscopy and MD simulations, finding that NHBs are inherently unstable, reforming ...