Lithium concentrations and Li isotope compositions are reported for natrocarbonatites and silicate lavas from Oldoinyo Lengai, Tanzania. Natrocarbonatites are characterized by very high Li contents (211–294 ppm) and a narrow range of δ7Li values between +3.3 and +5.1. These Li isotope compositions overlap with those reported for MORB and OIB and suggest that the natrocarbonatites reflect the Li isotopic composition of their mantle source. Co-genetic silicate lavas, covering a wide compositional spectrum, show no obvious isotopic fractionation as a function of igneous differentiation or liquid immiscibility. Primitive olivine melilitites (Mg#=58–70), considered to be parental magmas, contain 14–23 ppm Li and have δ7Li values of +2.4 to +4.4. A highly differentiated, peralkaline nephelinite (Mg#=12), likely to be related to the natrocarbonatites by liquid immiscibility, has about twice as much Li as the melilitite (57 ppm), but a similar isotopic composition (δ7Li=+3). In contrast, a phonolite with 15 ppm Li has a lighter Li isotope composition (δ7Li=−0.4), which may reflect assimilation of isotopically light lower crustal mafic granulites, a conclusion supported by radiogenic isotope data. Clinopyroxene and olivine separates from the silicate lavas have uniformly lower Li concentrations (3–15 ppm) and lower δ7Li values (δ7Li=−2.9 to −0.5) than the respective whole-rocks, with Δ7Liwhole-rock-mineral between 1.4 and 6.3. This difference between whole-rock and mineral data is interpreted to reflect diffusion-driven isotopic fractionation.