The dimensionality of proton transfer (PT) within the O-H..O intramolecular hydrogen bond of formimidol (O=CH-N=CH-O-H) is examined theoretically. The PT motion is assumed to depend explicitly only on the O-H..O in-plane vibrational coordinates. The remaining in-plane motions are assumed to be adiabatically responding to the PT motion. The corresponding vibrational (PT) energies exhibit sizable dependence on the adiabatic relaxation of the O-C-N and C-N-C angles advocating thus for their explicit consideration in a more quantitative description of the PT motion. (C) 2002 Elsevier Science B.V. All rights reserved.