The structures of the fluorescent products produced by the reaction of butylbiguanide, β-phenethylbiguanide, and N, N-dimethylbiguanide with 9, 10-phenanthraquinone in the presence of alkali were investigated. These biguanides gave a free base (IIIb) or ethanol adducts (IVb and Vb) of 2-alkylguanidino-1H-phenanthro [9, 10-d] imidazoles from alkaline solution and their hydrochloride monohydrates (IIIa and IVa) or dihydrochloride (Va) from acidic solution. The structure of these products was determined from their elemental analyses and nuclear magnetic resonance, mass, and high-resolution mass data.