The cocrystal hexamethylenetetramine–tridecanedioic acid (1/1) (HMT–C13), C₆H₁₂N₄·C₁₃H₂₄O₄, was investigated by single-crystal X-ray diffraction techniques at several temperatures during cooling and heating processes. Our results show the formation of two crystalline phases, separated by a large temperature phase co-existence between 290 and 340 K. Phase I, stable above 341 K, presents an orthorhombic structure described in the space group Bmmb, with one N₄(CH₂)₆·C₁₃H₂₂O₄ adduct in its asymmetric unit. Phase II, stable below 290 K, presents a monoclinic symmetry described by the space group P2₁/c, with two N₄(CH₂)₆·C₁₃H₂₂O₄ adducts in its asymmetric unit. The phase co-existence is observed both upon cooling and heating, and seems to be related to a complex domain-growth dynamic within the crystal.