Fluoro-substitution in the phenyl ring of some beta-(4-alkoxyphenyl)-dialdehydes markedly affects the mesophase behaviour of their copper(II) and oxovanadium(IV) complexes. Fluoro-substitution in the 2-position of the octyl and decyl copper(II) derivatives lowers melting and clearing temperatures, whereas substitution in the 3-position lowers only the former. Fluoro-substitution in the 5-position of the corresponding oxovanadium(IV) complexes usually lowers the melting point, but inhibits mesophase formation.