A stratigraphic sequence of scoria samples from a cinder cone in Vulsini, Italy, was studied for bulk rock and mineral chemistry and oxygen, strontium, and neodymium isotope chemistry. Hybridization processes between a relatively unevolved leucititi magma and an evolved trachytic magma of the low-K series are indicated by the mineral paragenesis, bulk rock trace element signature, mineral chemistry, oxygen isotope ratios of the crystals, and composition of glass inclusions. The assimilation of pelitic materials is suggested by the presence of cordierite-bearing glass blobs. Mixing calculations demand at least four end-members to explain the spectrum of compositions. Our detailed data show a variation in oxygen isotope composition comparable to the whole range of values obtained from Vulsini rocks. We propose a model of magmatic evolution in a chemically and thermally open system, in which evolved trachytic magmas have assimilated substantial amounts of crustal material with a heat supply from an underlying, primitive leucititic magma. Mixing of these strongly contaminated magmas with the underlying slightly evolved leucititic magmas led to the leucite tephrites. Limestone assimilation and absorption of high Δ18O geothermal waters might explain the poor correlation between stable isotope systematics and bulk rock chemistry. The Δ18O value of an unevolved leucititic magma represented in the scoria samples is slightly above the common range of mantle-derived magmas. ¿ American Geophysical Union 1989