Solid state synthesis of Li4Ti5O12 anode material for Li ion batteries typically results in products containing rutile TiO2 and Li2TiO3 impurities, and subsequent high calcination temperatures lead to particle growth that reduces capacity and rate ability. Here, the formation and growth of Li4Ti5O12 particles by a solid-state reaction using anatase TiO2 with various crystallite sizes and Li2CO3 is investigated by in situ high temperature powder X-ray diffraction (HT-PXRD) and thermal gravimetry-differential thermal analysis (TG-DTA). The combined data provide insight into the origin of the impurity phases and reveal that formation of Li4Ti5O12 from anatase TiO2 and Li2CO3 is a two stage process. Initially, TiO2 and Li2CO3 react to form monoclinic Li2TiO3, followed at higher temperature by a reaction with the remaining TiO2 to yield Li4Ti5O12. Four anatase TiO2 powders with different crystallite sizes (∼50 nm, ∼30 nm, ∼20 nm, and amorphous) were explored, and decreasing crystallite sizes causes a reduced initial reaction temperature. Using anatase with a crystallite size of ∼20 nm resulted in phase pure Li4Ti5O12 at the lowest temperature (800 °C). PXRD and TG-DTA results also revealed that Li4Ti5O12 decomposes to some Ti rich phases and probably Li2O when heated above 1000 °C.