Atrial fibrillation (AF) is the single most common post-operative complication after on-pump cardiac surgery, ranging from roughly 25% in CABG procedures, up to 65% in valve replacement procedures. The presence of AF in cardiac surgery patients doubles the morbidity amongst these patients, as well as raising the mortality rates. VitC is an electron donor for a number of enzymatic systems, reducing potentially harmful free radicals. In post-procedural AF, there is increased peroxynitrite concentration, which is a target for vitamin C. A randomized, double blind study will be conducted in order to establish the efficiency of vitamin C as prophylaxis for postoperative AF.