Ascorbic acid overload and vitamin B6 deficiency have been implicated in the development of hyperoxalemia in dialysis patients, but there is still disagreement about this. Hemodialysis patients who are exposed long-term hyperoxalemia may develop secondary oxalosis with an increased risk of cardiac, vascular, and bone disease, and thus may benefit from maintaining a low serum oxalic acid level. In 452 hemodialysis patients, the serum level of oxalic acid was 47.2 ± 22.9 μmol/l before and 16.9 ± 10.5 μmol/l after a 4-hour dialysis session, while the ascorbic acid levels were 39.0 ± 92.7 μmol/l and 6.5 ± 18.6 μmol/l, the glycolic acid levels were 7.3 ± 10.1 μmol/l and 0.6 ± 2.3 μmol/l, and the citric acid levels were 141.3 ± 54.7 μmol/l and 117.6 ± 37.2 μmol/l, respectively. Most patients (65.3%) had low serum ascorbic acid levels (<10 μmol/l) before hemodialysis. The serum level of oxalic acid [Ox] showed a significant positive correlation with the levels of ascorbic acid [AA], glycolic acid [Gly], and creatinine [Cre]: [Ox] = 21.711 + 0.181 x [AA] + 0.174 x [Gly] + 0.171 x [Cre], (all μmol/l, p < 0.05). In 124 dialysis patients, the 4-pyridoxic acid level was 8.9 ± 19.6 μmol/l before and 3.9 ± 8.8 μmol/l after dialysis, and it was not correlated with oxalic acid or glycolic acid. Most dialysis patients (65.3%) had low serum levels of ascorbic acid, but a subgroup of patients (12%) had high serum ascorbic acid levels (>100 μmol/l) associated with hyperoxalemia (88.2 ± 24.5 μmol/l). High-dose vitamin C supplementation may aggravate hyperoxalemia in hemodialysis patients, so attention should be paid to avoiding this risk.