Crack propagation thresholds in brittle materials are explained by consideration of the work done by the applied loading and that needed to create new surfaces as a crack propagates. The threshold mechanical energy release rate is shown to be a measure of the equilibrium surface energy of the material, dependent on the chemical environment. For applied loadings greater than those needed to maintain equilibrium the surface energy term introduces nonlinearities into the crack propagation characteristics. Any surface force or lattice trapping behavior at the crack tip will not influence the observed threshold provided the crack tip remains invariant on crack extension. A simple indentation/strength technique is demonstrated that permits the surface energy in the equilibrium state to be estimated. The technique is applied to the propagation of cracks in sapphire and the surface energy in water estimated as 1.42 J m−2, suggesting that the surfaces in water are stabilized by interactions stronger than van der Waals forces or hydrogen bonding alone. © 1986, Materials Research Society. All rights reserved.