This chapter briefly discusses the application, chemical properties and physical properties of hydrogen cyanide. It presents detailed information on the air analysis and analysis of aqueous samples of hydrogen cyanide. Hydrogen cyanide is released during combustion of wool, polyurethane foam and nylon, and produced when metal cyanides react with dilute mineral acids. It is soluble in water and alcohol, and dangerously toxic and highly flammable. During analysis of aqueous samples of hydrogen cyanide, aqueous samples are distilled directly without any acid treatment. During the air analysis of hydrogen cyanide, air is drawn through a solid sorbent tube containing soda lime. Hydrogen cyanide is liberated and collected in NaOH solution. It is analyzed by colorimetric or by ion selective electrode method. The method is originally developed for determination of plants, tissue, and toxicological substances. No precision and accuracy data are available for environmental samples of aqueous matrices.