Crystal structures of N-phenyl-N’-propylthiourea and the complex of N,N-bis(4,4’-difluoro-2-biphenyl)urea with N,N,N’,K-tetramethylurea have been determined by single-crystal X-ray diffraction. Both structures are stabilized by formation of intra- and intermolecular hydrogen bonds. The first compound adopts an anti-syn conformation whereas the second one has a syn-syn conformation. In both structures the nearest environment for the N atoms is planar. Structural similarities and differences between N-phenylureas and N-phenylthioureas are discussed together with their average geometry and relationships between changes of the structural parameters of these fragments. Hydrogen bonding and mesomerism involving ionic structures seem to be the dominant effects in changing the geometrical parameters of ureas and thioureas. The differences in the electronegativities of S and O atoms are responsible for a different mobility of pi-electrons in the examined fragments and in consequence for the differences in the variation of structural parameters.