A series of bis(pyrrolidone) and ε-caprolactam co-crystals of boric acid and 1,4-phenylenediboronic acid are reported. The high polarity of the lactam carbonyl group is highly complementary to the boronic acid hydrogen bond donor functionalities and co-crystals are based on cyclic boronic acid tape motifs, including an unusual donor–donor motif, with strong perpendicular interactions to the lactam functionalities. All of the co-crystals are able to act as a source of boric or boronic acid cross-linker in guar gelation.