The effect of phosphorous on the integrity of the thermally grown aluminium oxide layer formed on a platinum-aluminide coated a single-crystal nickel-base superalloy (CMSX-4) was studied through oxidation tests for varying exposure times followed by analysis using scanning electron microscopy and microanalysis. The samples were oxidized for up to 72 hours in pure oxygen (0.2 atm) at $1150^{circ}C$. Oxide spallation was shown on the surface even after short exposure (24 hour). The fraction of the spallation increased with time. Analysis revealed that phosphorous was associated with spalled areas in all conditions and is proposed to contribute to the failure by weakening to oxide/coating interface. A strong correlation was observed between phosphorous and chromium, as chromium was detected in phosphorous-rich areas.