The microstructure and phase transformation of a highly-branched polyethyleneimine/octadecanoic acid (PEI(OA)(1.0)) complex were investigated using a combination of DSC, XRD, optical polarised microscopy and temperature dependent FT-IR spectroscopy. A mesogen-free thermotropic liquid crystalline state was observed in a certain temperature region. The strong ionic interaction between -COO- (from OA) and -NH3+ (from PEI) and the hydrophobic interaction between the alkyl side chains contributes to the formation of a thermotropic liquid crystalline structure.