Manifestations of hydrogen bond donor:acceptor ratio and proton transfer in 2:1 and 2:3 cocrystals of phthalazine (PHZ) and phthalic acid (PA); (PHZ) 2 (PA), 1, and (PHZ) 2 (PA) 3 , 2, have been studied by single crystal X-ray analysis. While 1 is a simple hydrogen-bonded adduct of its constituents, 2 is a salt complex resulting from the transfer of acidic protons. 1 forms ''U-shaped'' molecular tweezers with two PHZ molecules hydrogen bonded to PA and separated by 6.8 9. The molecular tweezers are interdigitated via stacking interactions between PHZ units. 2 reveals a ''Z-shaped'' hydrogen-bonded pentamer consisting of a neutral PA, two monodeprotonated PA anions, and two protonated PHZ cations. The pentamers are held together through stacking and herringbone interactions between PA and PHZ units.