An improved thermodenuder is used to measure the volatility of secondary organic aerosol (SOA) produced during alpha-pinene/O-3 and alpha-pinene/NOx photooxidation. The thermodenuder allows a wide range of aerosol residence times in the heated zone compared to existing systems avoiding the complications due to slow mass and heat transfer processes. The performance of the thermodenuder was tested using mono-disperse ammonium sulfate particles. The volatility of SOA was investigated in the 50-220 degrees C temperature range. Almost 98% of the SOA volume generated from the alpha-pinene/O-3 reaction evaporated at 75 degrees C after 15.8 s in the heated zone. However, more than 50% of the particle mass did not volatilize at 100 degrees C when the residence time was reduced to 1.6 s. The SOA obtained from alpha-pinene/NOx photooxidation showed similar volatility characteristics even after 10 h of "aging" in the smog chamber. The measured remaining aerosol mass after the particles pass through the thermodenuder is quite sensitive to their residence time in the heated zone of the system, for residence times of the order of seconds. Interpreting the remaining aerosol mass as non-volatile even when the thermodenuder operates at temperatures above 200 degrees C may be erroneous if low residence times (less than a few seconds) are used. (c) 2007 Elsevier Ltd. All rights reserved.