Two crystalline color reaction products were isolated from the reaction mixture of butylurea with 1-phenyl-1, 2-propanedione in dioxane containing HCl. One of them (A) was found to be identical with 1, 3'-dibutyl-5, 5'-diphenyl-2, 2'-dioxo-4, 4'-diimidazolylmethane (I) which was isolated previously. The other one (C) was found to be isomeric with compound (B) which was also isolated previously. It was found that the only difference between B and C is the positions of their butyl side chains. In order to decide the positions of the side chains of B and C, the differences between the physico-chemical properties expected to be shown by 3, 3'-dibutyl-(II) and by 1, 1'-dibutyl-5, 5'-diphenyl-2, 2'-dioxo-4, 4'-diimidazolylmethane (III) were estimated theoretically. From the results of this estimation the structures II and III could be assigned to B and C, respectively.