The phase stability and phase transformation kinetics of Li1−xMPO4 olivines are critical to their performance as lithium storage electrodes. In this work, nanoscale (<100 nm primary particle size) Li1−xFePO4 and Li1−xMnPO4 are chosen as model systems for comparison with a coarser-grained LiFePO4 that exhibits a conventional two-phase reaction. The nanoscale materials first exhibit time and state-of-charge dependences of the electrochemical potential and structural parameters which show that stable two-phase coexistence is not reached. The evolution of structural parameters supports the existence of a coherency stress influenced crystal−crystal transformation. However, an additional response, the preferential formation of amorphous phase at nanosize scale, is identified. In Li1−xFePO4, at 34 nm average particle size, at least one amorphous phase of varying Li content coexists with the crystalline phases. In Li1−xMnPO4 of 78 nm particle size, the electrochemically formed delithiated phase is highly disorder...