Hafnium diboride (HfB2) is one of a family of ultra-high temperature ceramics (UHTCs) which are being considered for application in environments with a substantial heat flux such as hypersonic flight. In order to characterize transitions in the material response with heat flux and therefore predict the in-service behavior of UHTCs, a range of tests were conducted in which small cylindrical bars of HfB2 were laser heated using heat fluxes from 25 to 100 MW/m(2). After testing, the external damage as well as damage observable in cross sections through the cylinders was characterized using photography, optical, and scanning electron microscopy. Experimental results were compared with finite element modeling of the heat flow, temperature distribution, and phase transition. Heat flux rather than total deposited heat was found to be the strongest determinant of the way in which damage develops in samples; for lower heat fluxes, the main damage mechanism is oxidation, progressing to oxidation-induced melting and finally, at the highest heat fluxes, substantial ablation by melting irrespective of oxidation. The agreement between calculations and experimental observations indicates that such calculations can be used with confidence to guide the design of components.