The layered LiMO(2) (M=Co, Ni) compounds, which are of potential interest for Li-ion batteries, were synthesized at low temperatures by treatment under hydrothermal conditions of LiOH . H2O aqueous solutions containing powdered H(x)MO(2) phases. We studied the reaction mechanism and the influence of temperature, pressure, water dilution, and precursor ratio on the degree of progress of the ion exchange process. Single-phase LiMO(2) can be obtained in 48 h at 160 degrees C under an air pressure of 60 bars from an MOOH/LiOH . H2O/H2O mixture. The degree of advancement of the exchange reaction for M=Co was monitored in situ using an autoclave which allows the withdrawal of samples in the course of the reaction. From transmission electron microscopy coupled with x-ray diffraction studies we conclude that the reaction occurs by surface H+/Li+ exchange and is accompanied by a progressive breaking of the particles due to an interfacial collapse phenomenon. Infrared studies indicate that the LiCoO2 and LiNiO2 phases obtained are contaminated by carbonates that can more easily be eliminated in the case of LiCoO2 by water washing and post-heating treatments under primary vacuum at 200 degrees C for 2 days. Once the ion-exchange parameters are controlled, the LiMO(2) products exhibit electrochemical performances comparable to those of high-temperature made phases.