Catalytic pyrolysis of waste polymers over zeolitic catalysts has the potential to recover valuable hydrocarbons. Thermogravimetric analysis has been used as a tool to characterise the activity, regenerability and deactivation behaviour of zeolite US-Y in the degradation of high density polyethylene (HDPE). Deactivation of the catalyst occurs due to the deposition of coke. Analysis of the TGA results allowed a relationship between catalyst activity and coke content to be derived. The catalyst activity was found to fall exponentially with coke content.