The N-acyl thiourea complexes bis[N,N-diethyl-N′-(p-nitrobenzoyl)- thioureato]copper(II) (1a,1b) and bis(N,N-diphenyl-N′-benzoylthioureato) copper(II) (2a,2b) crystallize in each case in two modifications. X-ray structural analysis shows that 1a and 1b are cis-trans isomers. This is very unusual for N-acyl thioureato complexes because with exception of one platinum(II) complex up to now only cis complexes have been found. In contrast X-ray structural analysis of both forms 2a and 2b of the other complex shows no cis-trans pair. Both modifications are cis complexes. In solution both isomers of the copper(II) complexes are observable by EPR spectroscopy.