Pyruvate formate-lyase (PFL) is a glycyl radical containing enzyme that catalyzes the reversible CoA-dependent conversion of pyruvate into acetyl-CoA and formate. We have studied the catalytic mechanism of this enzyme by means of accurate quantum Chemical methods. It is shown that an overall homolytic radical mechanism is very feasible. In particular, the formation of a tetrahedral radical intermediate, by addition of thiyl radical to pyruvate, is supported by the calculated reaction energies and barriers. Furthermore, we propose that the thioester exchange between active site cysteine and CoA proceeds via a radical mechanism. This is made possible by the quenching of the formate radical by Cys418, and not Gly734, as previously proposed.