Epitaxial GaN layers were implanted with radioactive 181Hf ions in order to study the temperature dependence of the electric field gradient (EFG) at the site of the probes after rapid thermal annealing at 1273 K. Results are compared to recent measurements with the 111In probe that showed a surprising reversible increase of the fraction of undisturbed substitutional probe atoms at measuring temperatures above room temperature (Lorenz et al., Appl Phys Lett 80:4531, 2002). After implantation and annealing the majority of 181Hf atoms are incorporated into undisturbed substitutional lattice sites and experience an axial symmetric EFG with a quadrupole interaction frequency (QIF) of ≈336 MHz at RT and a low frequency distribution. At measuring temperatures between 20 and 1100 K the QIF increases linearly by ≈4% in good agreement with point charge model calculations taking into account the thermal lattice expansion. In contrast, measurements with the 111In probe showed a strong increase by ≈50% of the QIF above RT. Additionally, unlike for 111In, for measurements with 181Hf the substitutional fraction stays unchanged for elevated measuring temperatures.