In this paper, we report results from an investigation studying the purification of Co-filled carbon nanotubes (CNTs) using Thermogravimetric analysis (TGA). The as-grown CNTs were prepared using Microwave Plasma Chemical Vapour Deposition (MPCVD). Transmission electron microscopy (TEM), Fourier Transform Infrared (FTIR) and Raman spectroscopy were used to characterise the CNT samples. The CNTs produced by MPCVD were filled with cobalt and consisted of thick multi-walls. After TGA purification at 900 degrees C, 30 wt.% Co-filled CNTs remained in the TGA pan. However, while investigating the un-filled commercial CNTs (thin multiwalled), the sample completely burnt out at around 650 degrees C in the TGA furnace. The high thermal stability and the ability of thick-walled CNTs to act as an effective protective shield which prevents the oxidation of encapsulated cobalt have been demonstrated. (c) 2005 Published by Elsevier B.V.