During high-temperature oxidation of ZrB2-SiC composites, a multi-layer oxide scale forms with a silica-rich borosilicate liquid as the surface oxide layer. Here, a recently proposed novel mechanism for the high-temperature oxidation of ZrB2-SiC composites is further investigated and verified. This mechanism involves the formation of convection cells in the oxide surface layer during high-temperature oxidation of the composite. The formation of zirconia deposits found in the center of the convection cells is proposed here to be the consequence of liquid transport. The nature and deposition mechanism of the zirconia is reported in detail, using calculated phase equilibrium diagrams and microstructure observations of a ZrB2-15 vol% SiC composite tested at 1550 degrees and 1700 degrees C in ambient air for various times. The calculated phase equilibrium diagrams for the binary ZrO2-B2O3 system as well as the ternary B2O3-SiO2-ZrO2 system at 1500 degrees C are reported here to interpret these results.