One of the effects of endogenous NO is the formation of dinitrosyl iron complexes (DNICs), which, along with nitrosothiols, have been identified in all life forms, i.e., in the cells of bacteria, plants, and mammals [1]. Nitrosyl nonheme iron complexes have been identified by EPR as products of interaction of NO with some iron‐sulfur proteins and other iron-containing proteins [2‐5]. DNICs are intermediates in the iron-catalyzed decomposition and formation of S- nitrosothiols [6] and are considered as NO reservoirs and transporters in vivo. These results initiated the development of methods of synthesis and study of the physical and chemical properties of synthetic DNIC analogues [7] with the aim of their practical use as NO donors in medicine and biology [8, 9]. In the present paper, we report the results of the mass spectral and electrochemical studies of the products of hydrolysis of the tetranitrosyl iron thiosulfate complex Na 2 [Fe 2 ( µ -S 2 O 3 ) 2 (NO) 4 )] · 4 H 2 O (TNIC) in aqueous solutions. Study of the antimetastatic effect of the TNIC on three experimental models—melanoma B16, LL carcinoma, and AKATOL—showed that the complex inhibits metastatic growth. The combined administration of a low ineffective dose of the TNIC with cisplatin (or adriablastin) in leukemia P 388‐bearing mice leads to 100% survival among the laboratory animals [10]. Studying the TNIC decomposition products in solutions and determining the structure and properties of intermediates formed during hydrolysis are important for an understanding of molecular genetic mechanisms of action of exogenous NO donors of this class.