Ground-state interactions between Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) or Ru(bpz)(3)(2+) (bpz = 2,2'-bipyrazine) and phenol (PhOH) or monochlorophenols (ClPhOH) have been investigated in aqueous (D2O) solution by NMR. The upfield shifts of the resonances and the NOE difference spectra of the complexes in the presence of the phenols indicate the presence of offset face-to-face pi-stacking interactions between the phenol and the aromatic ligands. Electron withdrawal from the phenol ring by the monochloro substituent and less effective solvation by aqueous (compared to acetonitrile) media favor the interaction. The formation constant for the 1:1 Ru(bpy)(3)(2+)-PhOH complex is estimated to be similar to 0.01 M-1; K is smaller for Ru(bpz)(3)(2+) than for Ru(bpy)(3)(2+), and it increases for both complexes in the order PhOH < 4-ClPhOH < 3-CIPhOH < 2-CIPhOH.