The GdIII-complexes of DO3A-acetophenone and ortho- or para- hydroxyacetophenone ligands have been investigated to assess the effect of the presence of a phenol group on the relaxivity and on the water exchange rate of these potential MRI contrast agents. H-Bonding between the ortho-phenol(ate) groups and the water molecules involved in the dissociative exchange mechanism is shown to speed up the water exchange rate by stabilizing the eight-coordinate transition state.