Objective: To evaluate the physical compatibility between intravenous magnesium sulfate and potassium and sodium phosphate, a common electrolyte intravenous supplementation in pediatric intensive care units. Study design: Magnesium sulfate was mixed separately with potassium phosphate and sodium phosphate at ratios of 1:1, 1:4, and 4:1. Binary mixtures were prepared, in triplicate and under sterile conditions, by permuting the order of addition. The undiluted pure drugs were used as controls for possible sequence effects. Visual changes, turbidity, and pH were assessed immediately after mixing (baseline) and at 4 and 24 hours. Two observers performed visual changes by naked-eye visual inspection in order to search visible haze, particulate matter, gas formation, or color change. Turbidity was measured by nephelometry and incompatibility was defined as an increase of ≥0.5 nephelometric turbidity units (NTU) from baseline. pH was measured using a portable pH meter and incompatibility was defined as a variation of >1 pH unit during the observation period. Results: None of the admixtures exhibited visual changes or significant variations in turbidity (increases of ≥0.5 in nephelometric turbidity units) or pH (changes of >1 unit) during the observation period and neither compared with baseline. Conclusion: In this study, no visual changes were observed, and turbidity and pH evaluated by instrumental methods remained within acceptable limits and showed no significant variations from baseline, therefore no physical incompatibility between magnesium sulfate and potassium or sodium phosphate was found., Competing Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article., (© The Author(s) 2024.)