In this work, stresses generated during lithium transport through radio-frequency (rf) sputter-deposited Li1-deltaCoO2 films with different thicknesses were investigated by a double quartz crystal resonator (DQCR) technique. For this purpose, in situ resonant frequency changes of the Li1-deltaCoO2-coated AT- and BT-cut quartz crystals were first recorded along with the galvanostatic charge (lithium deintercalation) and discharge (lithium intercalation) curves obtained in a 1 M LiClO4-propylene carbonate solution. From the measured resonant frequency changes, the lateral stresses of the Li1-deltaCoO2 films were then estimated as a function of lithium stoichiometry, (1-delta). Compressive and tensile stresses were developed in the Li1-deltaCoO2 films during the lithium deintercalation and intercalation, respectively. The remarkable variation of compressive and tensile stresses with lithium stoichiometry appeared in a two-phase (a Li-poor alpha-phase and a Li-rich beta-phase) region. Compressive and tensile stresses decreased in absolute magnitude with increasing film thickness. The contribution of the electrostrictive stress to the total stress was theoretically calculated to be about 2.2 x 10(-3)%. From the extremely small contribution of the electrostrictive stress, it is strongly suggested that the stresses result mainly from the volume contraction and expansion of the Li1-deltaCoO2 films due to the lithium intercalation and deintercalation, respectively. Furthermore, the relaxation of the compressive stress was developed during the lithium deintercalation in a single alpha-phase region, causing the cracking of the Li1-deltaCoO2 films. (C) 2003 The Electrochemical Society.