Certain gases bearing elements such as arsenic, silicon, and sulfur, are known to cause Pt-Rh alloy degradation that leads to thermocouple failure at elevated temperatures. The detrimental effects of phosphorous, another element found in carbon feedstock, ores, and refractory bricks used in metallurgical and gasification processes, is, however, not well reported. In this work, gaseous phosphorous interactions with Pt-Rhx (x = 0–30 wt.%) alloys were studied isothermally and non-isothermally at various oxygen partial pressures. The analysis indicated material degradation was initiated by a combination of the Pt-Rh liquidus lowering and the formation of an intermediate phase at grain boundaries or gas/solid interfaces through two distinct phosphorous interdiffusion routes, critically depending on the amount of Rh in platinum. Based on the experimental result, a new partial isothermal section of the Pt-Rh-P system is proposed; along with unique transient degradation mechanisms of P in the Pt-Rh thermocouple alloys.