The thermal conductivity, thermal expansion, Young's Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB2, HfC0.98, HfC0.67, and HfN0.92 ceramics. The mechanical behavior of alphaHf(N) solid solutions was also studied. The thermal conductivity of modified HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820degreesC. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200degreesC for HfC0.98 to 1100degreesC for HfC0.67 ceramics. The transition temperature of HfB2 was 1100degreesC. Pure HfB2 was found to have a strength of 340 MPa in 4 point bending, that was constant from room temperature to 1600degreesC, while a HfB2 + 10% HfCx had a higher room temperature bend strength of 440 MPa, but that dropped to 200 MPa at 1600degreesC. The data generated by this effort was inputted into finite element models to predict material response in internally heated nozzle tests. The theoretical model required accurate material properties, realistic thermal boundary conditions, transient heat transfer analysis, and a good understanding of the displacement constraints. The results of the modeling suggest that HfB2 should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity. The comparison the theoretical failure calculations to the observed response in actual test conditions show quite good agreement implying that the behavior of the design is well understood. (C) 2004 Kluwer Academic Publishers.