Some interesting and unexpected effects observed during the in situ production of an Al-TiB2 metal matrix composite during the STS95, SPACE-SHUTTLE (1998) mission are described. It is shown that during the processing of the composites a Kirkendall effect in the liquid is observed. In contrast with the classical effect, this is due not only to the considerable diffusivity difference between the B and Ti in the Al matrix, but also to the chemical reaction taking place to form TiB2.