Background: Cardiovascular disease is the leading cause of death in patients with chronic kidney disease (CKD) and metabolic acidosis might accelerate vascular calcification. The T 50 calcification inhibition test (T 50 -test) is a global functional test analyzing the overall propensity of calcification in serum, and low T 50 -time is associated with progressive aortic stiffening and with all-cause mortality in non-dialysis CKD, dialysis, and transplant patients. Low serum bicarbonate is associated with a short T 50 -time and alkali supplementation could be a simple modifier of calcification propensity. The aim of this study was to investigate the short-term effect of oral sodium bicarbonate supplementation on T 50 -time in CKD patients. Material and Methods: The SoBic-study is an ongoing randomized-controlled trial in CKD-G3 and G4 patients with chronic metabolic acidosis (serum HCO 3 – ≤21 mmol/L), in which patients are randomized to either achieve serum HCO 3 – levels of 24 ± 1 mmol/L (intervention group) or 20 ± 1 mmol/L (rescue group). The effect of bicarbonate treatment on T 50 -time was assessed. Results: The study cohort consisted of 35 (14 female) patients aged 57 (±15) years, and 18 were randomized to the intervention group. The mean T 50 -time was 275 (± 64) min. After 4 weeks, the mean change of T 50 -time was 4 (±69) min in the intervention group and 18 min (±56) in the rescue group (β = –25; 95% CI: –71 to 22; p = 0.298). Moreover, change of serum bicarbonate in individual patients was not associated with change in T 50 -time, analyzed by regression analysis. Change of serum phosphate had a significant impact on change of T 50 -time (β = –145; 95% CI: –237 to –52). Conclusion: Oral sodium bicarbonate supplementation showed no effect on T 50 -time in acidotic CKD patients.