A new method has been developed to determine acetaldehyde, 2,3-butanedione (diacetyl) and 3-hydroxy-2-butanone (acetoin) in wine by solid-phase microextraction (SPME) and positive ion chemical ionization GC/MS analysis of O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBOA) derivatives. For SPME, a 65-microm PEG/DVB fibre was used; chemical ionization was performed with methane as reagent gas. The best analysis-time/sensitivity compromise was to perform the PFBOA reaction at 50 degrees C for 20 min, followed by 5-min SPME at the same temperature. Quantitative analysis was performed in SCAN mode using o-chlorobenzaldehyde as internal standard (IS), on the signal of the [M + H](+) ion at m/z 240 for acetaldehyde, 266 for acetoin (corresponding to the [M + H - 18](+) ion), 282 for diacetyl (protonated mono-derivatized compound), and 336 for IS. The accuracy and repeatability of the method were suitable for the study aims, and linearity was good in the range of concentration studied, with correlation coefficients of calibration curves 0.997, 0.998 and 0.988 for acetaldehyde, diacetyl and acetoin respectively. Due to the higher polarity of acetoin with respect to other two compounds, lower sensitivity in the detection of this compound was observed. By following the variation of the three carbonyl compounds, malolactic fermentations (MLF) were monitored in Merlot wines and this was carried out in the laboratory by two different bacteria strains.