The chemical stability and caesium retention properties of a nonstoichiometric compoundK2-xNix/2[NiFe(CN)6].nH2O (KNiFCN) was studied in different pH conditions. Different solutions with controlled pH were prepared. Leaching tests were done with chemical analysis of the solutions. The compound was characterized before and after exposure by chemical analysis, SEM-BSE, X-ray diffraction and infrared spectroscopy. The face-centered cubic (FCC) structure of KNiFCN without caesium was stable even in presence of 1 g/l HNO3 (pH equal 2) or NaOH (pH equal 13). In presence of caesium trapped on KNiFCN structure, it appears that the increase in caesium content change the KNiFCN structure, thus influencing the retention properties in KNiFCN according to the pH of the solution. The KNiFCN structure was decomposed when the NaOH concentration exceeds 1 g/l (pH equal 14), probably due to the hydrolysis of constitutional elements. A selection of cementitious matrix is the proposed to satisfy stability conditions of this compound.