Optical and electron metaliographic, X-ray diffraction, and electron microbeam probe studies of the interdiffusion of Cb-1 Zr alloy with type 316 stainless steel are reported. Extruded Cb-1 Zr alloy-type 316 stainless steel diffusion couples were heat treated at temperatures of 732, 871, 982, and 1093°C for time periods of US, 400, 900, 1400 hr. In no case was appreciable diffusion observed in specimens heat treated at 732°C for time periods chosen. Diffusion zones based on the systems iron-columbium and columbium-nickel-iron were observed in varying degrees in all other specimens. An activation energy for the rate-control ling process in the growth of the diffusion layers is reported. The growth rates of the observed zones were found to be retarded when these data were compared with data reported by other investigators for the interdiffusion of unalloyed columbiiim-type 316 stainless steel diffusion couples. Postulations arc made to explain this retarded growth rate and also to explain the growth of a nickel enriched columbium-iron-nickel-chromium intermetallic zone adjacent to the columbium.