This study of the sediments from Sacred Lake, Mount Kenya, demonstrates that δ13C analyses of individual lignin phenols can be used to assess past changes in the proportion of C3 and C4 plants within different plant groups in the lake catchment. Vegetation changes resulting from climatic and atmospheric changes since the last glacial maximum can be deduced from specific lignin phenols. Lignin, a major biopolymer in vascular plants, differs from n-alkyl lipids in (1) its higher specificity and more quantitative representation of vascular-plant input and (2) its major transport mechanism into the sediments by fluvial runoff, as opposed to the greater importance of eolian transport for leaf waxes and pollen. Lignin δ13C data support the hypothesis that atmospheric CO2 concentration is an important factor controlling the vegetation in tropical mountain ecosystems, and provide additional insights into past changes in vegetation structure. Specifically, the δ13C values of p-coumaric acid indicate the persistence of C4 grasses during the late glacial, when the lake level was lower than today, suggesting that aridity also played a role in controlling the abundance of C4 plants.