To improve the mechanical properties of copper coatings and bath stability, electroless deposition was carried out at an elevated temperature under aerated conditions. In this study, emphasis is placed on elucidating the effects of temperature and accumulated carbonate ions during the aeration on the mechanical properties of the electroless copper coatings. Electrochemical analysis using a quartz crystal microbalance electrode indicates that the mechanical properties are markedly improved by reducing the amount of adsorbed methylene glycol anion which seems to be an intermediate in the oxidation of formaldehyde. Changes in temperature and carbonate concentration alter the mechanical properties and crystal structure through influencing the amount of the adsorbed intermediate.