We studied the thermal decomposition of vanillin in a low-pressure pyrolysis microreactor by photoelectron photoion coincidence spectroscopy. The pyrolysis products were identified isomer-selectively by photoion mass-selected threshold photoelectron spectra (ms-TPES) based on Franck–Condon simulations and reference spectra. Methyl loss by homolytic C–O bond fission in the methoxy group is the initial unimolecular decomposition step. The primary intermediate, the 2-hydroxy-5-formyl-phenoxy radical (m/z 137), may undergo multiple decarbonylation steps and lose a hydrogen atom sequentially to finally form but-1-ene-3-yne at m/z 52. The ms-TPES and calculated reaction path energetics identified the product intermediates at masses of 109, 108, 81, and 80 amu. 3,4-Dihydroxybenzaldehyde (m/z 138) and 3,4-dioxocyclohexa-1,5-diene-1-carbaldehyde (m/z 136) peaks do not correspond to unimolecular decomposition products. Instead, they are formed in the hydrogen-transfer self-reaction of the primary methyl-loss fragment of vanillin at m/z 137. 3,4-Dihydroxybenzaldehyde is thermally more stable than vanillin, as also shown by its high activation energy to decomposition to m/z 137, 110 and 82 fragments. The lighter m/z 136 bimolecular product is prone to CO loss and its reaction path merges with the H-loss-containing unimolecular decomposition channel of vanillin. Further bimolecular reactions are also discussed. These mechanistic insights, especially the high (self-)reactivity of the methyl-loss intermediate and the resulting preponderance of bimolecular chemistry leading to inert dihydroxybenzaldehyde will guide process design involving vanillin and vanillin-like feedstocks.