This paper is devoted to the analysis of the single layer boundary integral operator C z for the Dirac equation in the two- and three-dimensional situation. The map C z is the strongly singular integral operator having the integral kernel of the resolvent of the free Dirac operator A 0 and z belongs to the resolvent set of A 0 . In the case of smooth boundaries fine mapping properties and a decomposition of C z in a 'positive' and 'negative' part are analyzed. The obtained results can be applied in the treatment of Dirac operators with singular electrostatic, Lorentz scalar, and anomalous magnetic interactions that are combined in a critical way.