Tracing the origin of iron cyanide complexes in the environment is important because these compounds are potentially toxic. We determined the stable isotopic compositions of cyanide carbon (C-CN) and cyanide-nitrogen (N-CN) in 127 contaminated solids and 11 samples of contaminated groundwater from coal carbonization sites, blast furnace operations, and commercial cyanide applications. Coal-carbonization-related cyanides had unique high mean delta C-13(CN) values of -10.5 +/- 3.5 parts per thousand for the solids and -16.1 +/- 1.2 parts per thousand for the groundwater samples, while the values for blast furnace sludge (-26.9 +/- 1.5 parts per thousand), commercial cyanides (-26.0 +/- 3.0 parts per thousand), and their corresponding groundwaters were significantly lower. Determination of delta C-13(CN) is a promising tool for identifying the source of cyanide contamination. However, for coal carbonization sites, historical research into the manufacturing process is necessary because a nonconventional gas works site exhibited exceptionally low delta C-13(CN) values of -22.7 +/- 1.7 parts per thousand. The delta N-15(CN) values for samples related to coal carbonization and blast furnaces overlapped within a range of +0.1 to +10.3 parts per thousand, but very high delta N-15(CN) values seemed to be indicative for a cyanide source in the blast furnace. In contrast, commercial cyanides tend to have lower delta N-15(CN) values of -5.6 to +1.9 parts per thousand in solids and -0.5 to +3.0 parts per thousand in the groundwater.