Introduction: Control of hyperphosphatemia in patients on dialysis remains a major challenge. Objective: This study evaluated predictors of serum phosphorus (sP) control among dialysis patients treated with noncalcium, oral phosphate binder therapy in a phase 3 clinical trial. Methods: Post hoc analyses were performed using data for patients with hyperphosphatemia who received 52 weeks of treatment with sucroferric oxyhydroxide (SFOH) or sevelamer carbonate (sevelamer). Patients were categorized into those who achieved sP control ( n = 302; defined as sP ≤ 5.5 mg/dL at week 52), and those with uncontrolled sP ( n = 195; sP >5.5 mg/dL at week 52). Because SFOH and sevelamer have previously demonstrated similar effects on chronic kidney disease-mineral-bone disorder parameters in this study, the treatment groups were pooled. Results: Average age at baseline was higher among sP-controlled versus sP-uncontrolled patients (56.9 vs. 53.4 years; p = 0.005). Baseline sP levels were significantly lower among sP-controlled versus sP-uncontrolled patients (7.30 vs. 7.85 mg/dL; p < 0.001), and sP reductions from baseline were significantly greater in the sP-controlled group (−2.89 vs. −0.99 mg/dL at week 52; p < 0.001). Logistic regression analysis identified higher baseline sP levels (odds ratio [OR] = 0.86, 95% confidence interval [CI]: 0.765–0.960), no concomitant active vitamin D therapy use (OR = 0.51, 95% CI: 0.328–0.804), and higher body mass index at baseline (OR = 0.96, 95% CI: 0.937–0.992) as significant predictors of uncontrolled sP. Conclusion: This analysis indicates that sP control may be more challenging in younger patients with high sP levels. Closer monitoring and management of serum phosphorus levels may be required in this population.