The curing behaviour and the development of physical properties of three commercial, silica filled epoxy/ anhydride resins that are used in solder ’flip-chip’ technology were investigated by differential scanning calorimetry, dynamic mechanical analysis (d.m.a.) and Fourier transform infra-red (FTi.r.) spectroscopy. In particular it was found that if the uncured resins were aged in the presence of moisture at 23 degrees C and then cured, the resulting properties such as glass transition temperature and rubbery modulus were significantly lower than the ultimate properties which developed if the resins were stored in the absence of water and then reacted. Also, under elevated temperatures and in the presence of high humidities or steam the hydrolysis of fully cured resins which had been previously stored in a dry state was detected by FTi.r. and d.m.a. measurements. The adhesion strength between solder balls and these epoxy underfill materials was found to deteriorate when aged under humid conditions.