Single-crystal EPR studies of the Fe-O-Fe dimer, [enH(2)] [{Fe(HEDTA)}(2)O]. 6H(2)O, have been performed at both X- and Q-band frequencies and room temperature. Resonances arising from the S' = 1, 2, and 3 states have been observed in the powder spectrum at Q-band frequency and analysis by spectral simulation has led to the zero-field splitting parameters: \D-1\ = 1.950 cm(-1), \E-1\ = 0.650 cm(-1), \D-2\ = 0.150 cm(-1), \E-2\ = 0.0195 cm(-1), \D-3\ = 0.570 cm(-1), and \E-3\ = 0.000 cm(-1). In addition, analysis of three orthogonal planes of single-crystal EPR data at both X- and Q-band frequencies has allowed the orientation of the zero-field splitting tensor with respect to the molecular geometry in the dominant (S' = 2) state to be determined. The largest principal value of the D tensor (D-zz) is found to lie approximately perpendicular to the plane of the Fe-O-Fe bridge; the nonlinear bridging angle is 165 degrees. The results obtained in this study are compared with those from a number of other studies. A pattern for the orientations of the zero-field splitting parameters in the dominant spin state (S' = 2) of the Fe-O-Fe dimers, so far studied by single-crystal EPR spectroscopy, emerges.