Unstabilized poly(ether block amide) (PEBA, PEBAX) of the type used in angioplasty balloons was aged at temperatures of 18-858C for periods of up to 313 days. Since PEBA is used in safety critical applications, understanding the effects and underlying mechanisms of ageing and the validity of accelerated ageing protocols is vital. Mechanical testing indicated an increase in modulus during ageing consistent with embrittlement. An activation energy (E a ) of 47.3 kJ mol 21 was calculated over the temperature range 18-458C and increased deviation from linear Arrhenius behavior observed above 458C. This value of E a would result in the commonly applied 108C rule over estimating shelf life.