Li4Ti5O12 (LTO) is considered to be a "zero strain material" that shows negligible stresses upon electrochemical cycling. Here we report on the stress evolution of this material for various voltages and lithiation/delithiation rates. The features in the stress signal are correlated to the coexistence of the two phases Li4Ti5O12 and Li7Ti5O12 as well as to structures that form during under- and overlithiation. The formation of the under- and overlithiated structures occurs as soon as the potential deviates from its signature plateau at 1.56 V vs. Li/Li+, where the two phases Li4Ti5O12 and Li7Ti5O12 coexist. In relaxation experiments time dependent phenomena in the under- and overlithiated states are investigated. Compared to other electrode materials, the stress level changes are moderate during operation at or close to the plateau voltage, but a large stress forms particularly for potentials below 1.0 V vs. Li/Li+. This high stress accumulation which would be an adverse effect of attempts to increase the capacity by further lithiation beyond the Li7Ti5O12 state is likely due to the formation of a lithium-rich phase that may compromise the reliability of this material at very low potentials. (C) 2013 Elsevier B.V. All rights reserved.