Alumina + zirconia fiber (PRD-166)/glass (N51A) matrix composites were fabricated with and without a tin diozide barrier layer. A qualitative comparison of the elemental distribution was made between the two composite systems by secondary ion microscopy. Secondary ion mass spectrometry would appear to be especially useful to obtain the distribution of light elements such as B, Na, Cl, etc. in ceramic and glass composites. Our study of PRD-166/glass and PRD-166/SnO2/glass composites shows that tin dioxide serves as an effective barrier between this (alumina + zirconia) fiber and the silica-based glass. © 1993.