A central task of theoretical physics is to analyse spectral properties of quantum mechanical observables. In this endeavour, Mourre's conjugate operator method emerged as an effective tool in the spectral theory of Schrödinger operators. This paper introduces a novel class of examples amenable to Mourre's method, focusing on the spectra of energy-momentum operators in relativistic quantum field theory. Under the assumption of Lorentz or dilation covariance and the spectrum condition, we provide new proofs of the absolute continuity of the energy-momentum spectra, showcasing the efficiency of Mourre's method in the relativistic setting.