A new interpretation of the reaction mechanism between active metal thin-film filler and ceramic substrate is proposed. The authors predict the possibility of prebonding reactions, prior to melting of the filler, at the interface of the system described above. To prove this, solid-state reactions of Ag–Cu–Ti thin films on sapphire substrates have been studied with Auger electron spectroscopy (AES) and x-ray diffraction (XRD). Reaction process and products have been clarified at the temperature just below the melting point of the filler. It is evident that Cu3Ti3O (diamond structure of Fd3m) is formed by the reaction between Cu3Ti and O which results from the reduction of sapphire. It seems that Cu3Ti3O contributes to bonding between metals and sapphire as an intermediate phase.