Mesomorphic structures of supramolecular systems based on poly(l-vinylimidazole) (PVI) and alkanoic acids with chain lengths (n) of 10-18 carbon atoms have been studied. FTIR studies show the existence of hydrogen-bonding interaction and a low level of ionic interaction. POM measurements show that all the complexes are mesomorphic, and the isotropization temperature increases with increasing chain length of the acid. In addition, the isotropization temperature increases with decreasing acid content in the complex. DSC studies show that, besides isotropization transition, two melting transitions exist in complexes containing alkanoic acids with chain lengths n greater than or equal to 1-2. On the basis of XRD studies at room temperature and elevated temperatures, the complexes are grouped into two types: melted lamellar liquid crystal phase with interdigitating layer structure; crystallized lamellar phase with partial interdigitating layer structure. For PVI(PA)(x) and PVI(MA)(1.0) (MA: myristic acid, n = 14) complexes, these two types of structures are interconvertible upon heating/cooling with a change in layer thickness of about 10 Angstrom. As shown by the present studies, the transition temperature and the thickness of layer can be tailored by varying the acid type and by changing the acid content in the complex.