Activated carbon has been milled for up to 1000 h in a laboratory-scale tumbling ball mill under a vacuum. Thermogravimetric (TG) analysis of the powder in argon showed an increasing mass loss with milling time indicating the presence of chemisorbed gas. TG Fourier transform infrared spectrometry showed the gas was a mixture of water, CO and 2 an unidentified gas (probably oxygen). BET surface area measurements showed a decreasing surface area with milling time, however, this was shown to be massively in error for the longer milling times due to the presence of the chemisorbed gas. The area occupied by the chemisorbed gas increased from 40 to 80% of the true surface area which was almost constant at 2 21 1258627 m g for all three powders. These results show that extremely large errors may be made when using BET analysis to determine the surface area of powders, especially those where the surface activity is substantially increased during processing.