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 340K. Phase I, stable above 341K, 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 290K, 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.