IRON-CHROMIUM ALLOYS PLATES WERE HEATED AT 840 K IN FUSED SALT BATHS WITH HIGH ALKALINE CYANATE CONTENTS. LONG-TERM HEAT TREATMENT UNDER THESE CONDITIONS LED TO THE FORMATION OF A(FE,CR) (C,O,N) PHASE ON THE SURFACE OF THE SPECIMEN. THE CHARACTERISTICS OF THE PHASE WERE INVESTIGATED BY X-RAY DIFFRACTION ANALYSIS, MICROPROBE DETERMINATION AND CHEMICAL ANALYSIS. THE PHASE HAS A CUBIC (NACL TYPE) UNIT CELL WITH A LATTICE PARAMETER INTERMEDIATE BETWEEN THOSE OF CRN AND WUSTITE.AS IN THE CASE OF WUSTITE, THE STRUCTURE IS IRON-DEFICIENT AND BECOMES MUCH MORE SO AS OXYGEN IS ADDED.