A very simple, accurate, fast, selective and sensitive assay of cyanide based on its reaction with 2,2-dihydroxy-l,3-indanedione at basic pH is proposed. As little as 0.01 mug ml(-1) of cyanide can be determined. The molar absorptivity may reach 5.1-8.0x10(4) l mol(-1) cm(-1) depending on the reaction conditions. Thus, 1 ml of sample solution is mixed with 500 mul of 5 mg ml(-1) solution of 2,2-dihydroxy-1,3-indanedione monohydrate in 2% sodium carbonate. The absorbance of the purple color is measured at 510 nm in 1-cm glass cuvettes, 10-15 min after mixing the reagents. The procedure could also be used to identify free CN(-) in natural waters and hydrocyanic acid in the environment.