Critical Dirac operators are those which have eigenfunctions andor resonances for E=m. We estimate the behavior of the generalized eigenfunctions of critical Dirac operators under small perturbations of the potential. The estimates are done in the L∞ -norm. We show that for small k, the generalized eigenfunctions are in leading order multiples of the respective eigenfunctions andor resonances. We also estimate the k -derivatives which are important for estimating decay. The method also applies for other differential operators (for example, Schrödinger operators). © 2007 American Institute of Physics.