The phase transformation of delta Al2O3 occurring in Saffil fibres during their infiltration with molten Mg-8 wt% Li alloy was studied by secondary ion mass spectroscopy, X-ray diffraction and infrared spectroscopy methods. It has been shown, that lithium penetrates very quickly into the whole fibre volume, attaining up to Li/Al approximate to 0.25-0.30 ion ratio. The metastable spinel-like compound, gamma(Li), was formed by incorporation of Li+ ions into the delta Al2O3 lattice in which the basic spinel structure unit has been assigned by the formula Al-8 [Al-(40-x/3)square((8-2x)/3)Li-x]O-32. During long-term annealing, a further transformation gamma(Li) --> LiAl4O8 proceeded, and LiAlO2 aluminate was also identified in Saffil fibres with high Li/Al concentration ratio values. In parallel with lithium, magnesium also penetrated the Saffil fibres within an infiltration period; however, the incorporation of magnesium into the spinel lattice has not been observed.