Chemically compatible 50 vol% Al2O3-50 vol% YAG (yttrium aluminum garnet, Y3Al5O12), 50 vol% Al2O3-50 vol% mullite, and 50 vol% Al2O3-50 vol% NiAl2O4, in situ, duplex composite powders were fabricated by a chemical synthesis method. Both amorphous and crystalline powders were synthesized. The physical and mechanical properties of YAG and in situ, solution derived composites made using these powders were analyzed. Mechanically mixed, composite powders were made by a conventional, ball-milling technique. The properties of the composites made from in situ versus mechanically mixed powders were studied. Compared with pellets made from crystalline powder, amorphous powder-derived pellets had about a twofold higher sintering shrinkage. The crystalline powder-derived YAG pellet sintered at 1650 degrees C for 10 h had a three-point bending strength of 244 +/- 3 MPa. A 50 vol% Al2O3-50 vol% YAG in situ composite achieved an average bending strength of 361 +/-19 MPa after sintering at 1700 degrees C for 5 h. Grain size measurements indicated a mutual grain growth retardation effect of alumina and YAG phases in the composites. The better properties of in situ composites compared with mechanically mixed composites were attributed to the intimate mixing, at a molecular level, during the processing of in situ composites. The possible formation of a glassy phase in the Al2O3-mullite in situ composite heat treated at higher temperatures was discussed. The physical and mechanical properties of alumina-mullite and Al2O3-NiAl2O4 in situ composites were investigated.