10 Real time monitoring of volatile organic compounds (VOCs) using a Proton-Transfer Reaction Mass Spectrometer was performed at the Mauna Loa Baseline Station (19.54N, 155.58W) in March/April 2001 (March 23, 2001–April 17, 2001). Mixing ratios for methanol, acetone, acetonitrile, isoprene and methyl vinyl ketone (MVK) plus methacrolein (MACR) ranged between 0.2 and 1.8, 0.2 and 1, 0.07 and 0.2, <0.02 and 0.3, and <0.02 and 0.5 ppbv, respectively. Biomass burning plumes transported from South-East Asia and the Indian Subcontinent across the Pacific influenced part of the measurement campaign. Acetonitrile/CO and acetone/acetonitrile ratios in these cases were 1.5 × 10 −3 to 2.5 × 10 −3 and 2–5 ppbv/ppbv, respectively. Overall Asian outflow events were not as frequent during Spring 2001 as in previous years. Methanol did not show significant correlation with CO, acetonitrile, and acetone. The abundance of acetone and CO seemed to be influenced but not dominated by biomass burning and domestic biofuel emissions. (Int J Mass Spectrom, in press) 11 12 13 14 15 16 17 18 19 © 2002 Published by Elsevier Science B.V.