We demonstrate heterogeneous chemistry between Li and anatase TiO2 nanoparticles under UHV. The reduction of TiO2 upon formation of lithium oxide proceeds via two different schemes: one that reduces Ti(4+) to Ti(3+) and one that reduces Ti(4+) directly to Ti(2+). The second scheme sets in only after a critical degree of reduction (i.e. Li amount) has been reached (Li/Ti = 0.28) and is associated with restructuring of the film. Two films with different morphologies were compared and the results demonstrate that the reaction between Li and larger TiO2 structures (30-50 nm) is kinetically restricted while such effects were significantly less prominent for small particles (10 nm).