The phthalocyanine salt [ZnPc(NMe3)4]I4 was synthesized from 4-nitrophthalonitrile using a three-step procedure. The interaction of [ZnPc(NMe3)4]4+ with calf thymus DNA (CT DNA) has been investigated by UV-vis and fluorescence spectrometric methods. [ZnPc(NMe3)4]4+ exists in a non-monomeric form, proposed to be a dimer, in phosphate buffer solution (pH 6.82). Spectral changes show that in the presence of high concentrations of CT DNA added to the solution, [ZnPc(NMe3)4]4+ is bound in a monomeric state with evidence suggesting it is located in a DNA groove. At lower concentrations of DNA there is evidence of stacking of non-monomeric [ZnPc(NMe3)4]4+ onto the DNA. Two intrinsic binding constants for the interaction of [ZnPc(NMe3)4]4+ with CT DNA, 1.33 × 105 and 2.56 × 104 M-1 have been obtained. Electrostatic binding is shown to play an important role in the interaction of [ZnPc(NMe3)4]4+ with nucleic acids.