Examination was made on the formation of alkylmercury compounds by the sunlight or blacklight irradiation of mercuric chloride together with acetic acid, propionic acid, methanol, or ethanol in water. Alkylmercury compounds formed in the aqueous solution were extracted and subjected to qualitative and quantitative analyses by thin-layer chromatography, mercury, analyzer, and gas chromatography. As a result, it was found that methylmercury compound was the sole one formed in the aqueous solution of mercuric chloride in the presence of acetic acid, methanol, or ethanol with either sunlight or blacklight, while both ethylmercury and methylmercury compounds were formed in the presence of propionic acid. Teh quantitative analysis showed that fatty acids contributed remarkably to the formation of alkylmercury compounds in comparison with alcohols.