In the smooth scattering theory framework, we consider a pair of self-adjoint operators H 0 , H and discuss the spectral projections of these operators corresponding to the interval ( − ∞ , λ ) . The purpose of the paper is to study the spectral properties of the difference D ( λ ) of these spectral projections. We completely describe the absolutely continuous spectrum of the operator D ( λ ) in terms of the eigenvalues of the scattering matrix S ( λ ) for the operators H 0 and H. We also prove that the singular continuous spectrum of the operator D ( λ ) is empty and that its eigenvalues may accumulate only at “thresholds” in the absolutely continuous spectrum.