Both the 1:1 and 2:1 molecular adducts of 4-methylimidazole (4-MeIm) and terephthalic acid (H2TPA) are organic salts, viz. C4H7N2+.C8H5O4-, (I), and 2C4H7N2+.C8H4O4(2-), (II), respectively. The component ions in (I) are linked by N-H...O and O-H...O hydrogen bonds into continuous two-dimensional layers built from R6(4)(32) hydrogen-bond motifs running parallel to the (100) plane. These adjacent two-dimensional layers are in turn linked by a combination of C-H...O, C-H...pi and pi-pi interactions into a three-dimensional network. In the crystal structure of (II), with the anion located on an inversion centre, only N-H...O hydrogen bonds result in two-dimensional layers built from R8(8)(42) hydrogen-bond motifs running parallel to the (102) plane. Being similar to those in (I), these layers are also linked by means of C-H...O, C-H...pi and pi-pi interactions, forming a three-dimensional network. This study indicates that, on occasion, a change of the reactant concentration can exert a pivotal influence on the construction of supramolecular structures based on hydrogen bonds.