We identified the effects of vegetation changes, and aerobic and anaerobic decay on the lignin composition in the Penido Vello peat record (Galicia, Spain). The ombrotrophic part of this peat record was dominated by graminoids and has significant contributions of ericoids at some depths. The organic matter (OM) of different peat fractions (bulk, NaOH-extractable fraction, and non-extractable residues) of 15 samples from the upper meter was analysed with pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). In addition, the dominant plant species were analysed, including Carex durieui, Agrostis curtisii, Molinia caerulea, Deschampsia flexuosa, Festuca rubra, Eriophorum angustifolium, Erica mackaiana and Calluna vulgaris, and their lignin composition compared to that of the peat OM. The high abundance of guaiacol and 4-formylguaiacol in fresh plant tissue compared to peat OM suggests that in addition to p-coumaric and ferulic acid (which are abundant in graminoids), other non-lignin phenolic monomers are contributed by graminoid species. For the non-lignin phenolics, graminoids differed from ericoids in the high abundance of ferulic acid (4-vinylguaiacol), while p-coumaric acid (4-vinylphenol) showed high and similar abundances in ericoids and graminoids. This result suggests that ratios between p-hydroxyphenyl (or p-coumaric acid) and other lignin moieties in (pyrolysates of) peat cannot be used as source indicator. Comparison of plant and peat fractions using factor analysis allowed a distinction between the effects of source (plant identity) and decay on the lignin composition of the Penido Vello peat, and different stages of decomposition were identified. Preferential decay of guaiacyl over syringyl moieties was found for the first stage of decay. This preferential decay is probably related to the large abundance of guaiacyl moieties in easily degradable non-lignin phenolics. Preferential decay of syringyl moieties occurred during subsequent aerobic decay.