Objectives: This article aims to explain the altered oxida- tive status and thiol/disulfide homeostasis before and after surgery in children with congenital heart disease (CHD). Methods: Blood samples were taken from the patients (n=50) before the operation (baseline), at the 1st hour, and at the 24th hour after the operation. Thiol-disulfide levels, total antioxidant status (TAS), total oxidant status (TOS), oxida- tive stress index (OSI), myeloperoxidase (MPO), cerulo- plasmin, albumin, ischemia-modified albumin (IMA), and prolidase activities of all samples were measured. Pre- operative oxygen saturation (SaO2) values and neutrophil/ lymphocyte ratios (NLR) were also measured. Results: Before the operation, TOS, OSI, MPO, cerulo- plasmin, IMA, NRL, and disulfide levels were higher in the cyanotic group than in the acyanotic group. When the indicated three different time points were compared, the TOS, OSI levels, and MPO activities of the 24th hour were significantly lower than the baseline and 1st hour. In comparison, ceruloplasmin levels of 1st hour were signifi- cantly higher than of the baseline and 24th hour. Native thiol and total thiol levels in the baseline group were significantly lower than in the 1st and 24th hours. Disulfide levels of the 24th hour were significantly lower than of the baseline. Conclusions: The operation leads to changes in the thiol- disulfide balance and oxidant status in CHD.