Abstract A new method for the preparation of K 3 [ReO 2 (CN) 4 ] from [ReO 2 py 4 ]Cl·2H 2 O is described. The acid association constants of [ReO 2 (CN) 4 ] 3− have been determined by Bjerrum's method using Calvin-Wilson titration technique. The violet colour formed by the addition of acid to a solution of [ReO 2 (CN) 4 ] 3− has been shown by the method of continued variation to be due to a monoprotonated species [ReO(OH)(CN) 4 ] 2− . By the action of dilute hydrochloric acid on K 3 [ReO 3 (CN) 4 ] a deep pink crystalline substance of the composition ReO(CN) 3 · 2H 2 O has been isolated. This is acidic and very weakly paramagnetic. On the basis of interesting findings obtained in potentiometric and conductometric titrations of the substance with alkali, it has been formulated as H[ReO(OH)(H 2 O)(CN) 3 ] which remains in equilibrium in solution with its dibasic acid form H 2 [ReO(OH) 2 (CN) 3 ]. The dissociation constants of the acid have been determined. Various salts of both the mono-and dibasic acids, viz. M I ReO(OH)(H 2 O)(CN) 3 ] and M 2 I [ReO(OH) 2 (CN) 3 ] have been isolated. The i.r. spectra of the acid and also of the monopotassium salt gave both OH and H 2 O bands, while the spectra of the dipotassium salt gave OH, but no H 2 O band. The molecular conductance of the salt solutions also supports the formulae suggested. On strong desiccation and heating to 300°C the acid gave the polymeric substances H 2 [O(H 2 O)(CN) 3 Re·O·Re(CN) 3 (H 2 O)O] and O(CN) 2 Re·O·Re(CN) 2 O respectively.