Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through the metal-insulator transition (MIT). Upon cooling, the modulus presents a narrow dip at the MIT followed by an abrupt stiffening of ∼6%. The anomaly is reproducible between cooling and heating in Nd0.65Eu 0.35NiO3 but appears only as a slow stiffening during cooling in undoped NdNiO3, in conformance with the fact that the MIT in RNiO3 changes from strongly first order to second order when the mean R size is decreased. The elastic anomaly seems not to be associated with the antiferromagnetic transition, which is distinct from the MIT in Nd 0.65Eu0.35NiO3. It is concluded that the steplike stiffening is due to the disappearance or freezing of dynamic Jahn-Teller (JT) distortions through the MIT, where the JT active Ni3 + is disproportionated into alternating Ni3+δ and Ni3-δ. The fluctuating octahedral JT distortion necessary to justify the observed jump in the elastic modulus is estimated as ∼3% but does not have a role in determining the MIT, since the otherwise-expected precursor softening is not observed. © 2011 American Physical Society.