The crystal structures of N,N＇-diphenylguanidine (1) and its N-methylated derivatives were investigated, and the conformational properties of these molecules were utilized to construct water-soluble oligomers with multilayered aromatic structures. N,N＇-Diphenylguanidine (1) afforded two types of crystals, chiral (P2(1)2(1)2(1)) and racemic (P2(1)/c), upon recrystallization from EtOH. In both crystals, 1 exists in the (E,Z) conformation, in which one C-N bond (length : 1.28-1.30 Angstrom) attached to a phenyl ring shows double-bond character. In contrast, N,N＇-dimethyl-N, N＇-diphenylguanidine (4a) exists in the (Z,Z) conformation with the two aromatic rings facing each other. As judged from the crystal structures of several N-methylated compounds, the conformational preferences of diphenylguanidines appear to be related to those of aromatic anilides. N,N,N＇,N "-Tetramethyl-N＇,N "-diphenylguanidinium iodide (6) afforded chiral crystals, like 1 and N-methyl-N,N＇-diphenylguanidine (2). The absolute structure of each enantiomeric propeller conformation of 6 was determined by X-ray analysis using the Bijvoet difference method. The Z-conformational preference of 4 allowed us to synthesize oligomeric di- or tetraguanidines (9-12) which have multilayered aromatic structures both as a crystal and in organic and aqueous solvents.