The Morita-Baylis-Hillman (MBH) reaction allows chemists to form new sigma C-C bonds in a single-step straightforward manner and thus to construct densely functionalized molecules for further chemical manipulation. Using electrospray ionization for transferring ions directly from solution to the gas phase, and mass (and tandem mass) spectrometry for mass and structural assignments, new key intermediates for the rate-determining step of the MBH reaction have been successfully intercepted and structurally characterized. These ESI-MS data provide experimental evidence supporting recent suggestions, based on kinetic experiments and theoretical calculations, for the dualist nature of the proton-transfer step of the MBH mechanism.743031-7