In addition to arsenic, silicon, and sulfur; phosphorous is also known to cause severe degradation of platinum-based alloys, leading to thermocouple failure or erroneous EMF output during industrial operation at elevated temperatures. The alloy behaviors in contact with phosphorous, an element commonly present in gasification carbon feedstock, industrial ores, and certain refractory bricks used in glass, metallurgical and gasification processes, are, however, not well documented. In this work, diffusion of phosphorous from a gaseous source that interacts with a Pt-30 wt.% Rh alloy was isothermally investigated at 1285 K. Interdiffusion of P and Rh through the fcc alloy led to the formation of two layers with distinguishable microstructures consisting of (Pt, Rh)₂P, liquid, platelets, and fcc. The Pt-Rh alloy corrosion mechanism will be explained using microstructural evolutions.