A complex ion thought to be [Fe(NO₂)(CN)₅]^5– has been prepared by reduction of [Fe(NO₂)(CN)₅]^4– with sodium borohydride. The e.s.r. spectrum has g_x 1·9993, g_y 1·9282, and g_z 2·0080, and A_x(¹⁴N)–32G, A_y 10G, and A_z 15G. These parameters are in agreement with expectation for the ion if the unpaired electron is in an orbital of b₁ symmetry, being about 75% localised on the NO₂ ligand, which thus resembles NO₂^2–. This is the same species as that detected after reduction of sodium nitroprusside by aqueous sodium sulphide. The equilibrium [Fe(CN)₅NO]^3–+ 20H^–⇌[Fe(NO₂)(CN)₅]^5–+ H₂O is invoked to explain the effect of pH on the system. This contrasts with the conclusion of van Voorst and Hemmerich who postulated an equilibrium of the form [Fe(CN)₅NOH]^2–+ OH^–⇌[Fe(CN)₅NO]^3–+ H₂O.