Energy-dependent fine-structure effects (FSEs) on the reactivity associated with perturbations in the density and temperature of resonance materials are investigated using a simple single-resonance model Upper and lower bounds to these spectral effects are derived, and parametric studies are performed as a function of the background cross section and resonance structure of the unperturbed assembly and of the type and relative magnitude of the perturbation. It is found that spectral FSEs can be significant even for infinitesimal density or temperature perturbations. The capability of different perturbation theory formulations to account for these FSEs is investigated. The connection between the spectral FSEs and the disagreement of the calculated to the experimentally determined material and Doppler reactivity worth of fuel isotopes in fast critical experiments is also discussed.