Borosilicate glass-alumina composites with (1 x) Glass + x Al2O3 (x = 0, 5, 10, 25 vol.%) were prepared and the effect of Al2O3 addition on the structural, electrical and thermal characteristics was investigated. XRD patterns revealed the presence of cristobalite (SiO2) in sintered borosilicate glass and that the addition of Al2O3 hinders cristobalite formation. This behavior is due to the diffusion of some Al3+ ions from alumina to glass, which leads to changes in glass structure and composition as identified by SEM/EDS. Cristobalite was undetected in composites containing 10% Al2O3 that attained the lowest thermal expansion coefficient value (4.6 106 C1). Conductivity (dc and ac) increased with the amount of Al3+ ions present in the glass structure as modifiers and formers. Dielectric constant values, in the range 5.0-7.2, increased with Al2O3 addition and the values of loss tand (1.5-2.1 102) indicate that these materials are good insulators.