We deduce information about the geometry of the hydrogen bond between hydroquinone and ammonia from an analysis of the fully resolved S-1<--S-0 fluorescence excitation spectrum of the 1:1 complex in the gas phase. The complex is planar in both electronic states, with NH3 forming a nearly linear hydrogen bond to one of the hydroxy hydrogen atoms of hydroquinone. The O-H ... N heavy atom separation is R=2.85 Angstrom and the barrier to internal rotation of the NH3 group about its C-3 axis is V-3 = 35.5 cm(-1) in the S-0 state. Excitation of the complex to its S-1 state decreases the heavy atom separation to R = 2.77 Angstrom and increases the torsional barrier to V-3 = 58.8 cm(-1). These changes are a direct consequence of the increased acidity of hydroquinone in its S-1 state.