The interfacial reaction at the whisker-matrix interface in a K2O . 6TiO(2)/6061Al composite was investigated by high-resolution transmission electron microscopy. Magnesium segregation and titanium enrichment at the whisker-aluminium interface was revealed by energy-dispersive X-ray analysis. It was shown that TiO and MgTi2O4 layers and MgAl2O4 particles were formed at the whisker-aluminium interfaces in the composite during the manufacturing of the composite. The thickening of the reaction layer after T6 treatment may result in the decrease of the bending strength of the composite. Specific orientation relationships between MgTi2O4 and TiO, and also between TiO and K2O . 6TiO(2) whiskers were also found.