Epithermal nonabsorption probabilities for neutron absorbers are important parameters in the evaluation of control rod worths. In this article, the average epithermal nonabsorption probability () for hafnium slabs is calculated by two different methods: (1) Within the framework of a specific two-dimensional, three-energy-group calculational scheme, an empirical value of is determined which gives agreement with experimental rod worths. (2) Based on available microscopic cross-section data for the hafnium isotopes, the value of is calculated and compared with the empirical value obtained above. The values of obtained by these different methods for 0.1 in. and 0.2 in. thick slabs agree to within 10 per cent, thus verifying the physical reasonableness of the empirical probabilities.