Background: Although several dexamethasone phosphate preparations are commercially available and frequently administered with octreotide acetate, their compatibility remains unknown. Aim: We investigated the effect of pH and sodium bisulphate on the stability of octreotide acetate. Measurement design: Octreotide acetate percentage was measured 3 and 10 days after it was mixed with 2 dexamethasone phosphate preparations containing different concentrations of sodium bisulphate as an additive, and in one that did not contain sodium bisulphate. Solutions were also analysed after they were prepared using phosphate buffer to achieve pH values of 4.0, 7.0, and 9.0. The initial octreotide acetate concentration was 41.7 g/mL. High-performance liquid chromatography was used for measurement. Results: The octreotide acetate percentage in the mixture with dexamethasone phosphate without sodium bisulphate was maintained at 95% for up to 10 days. However, mixing octreotide acetate with the other 2 agents resulted in a significant decrease to 85%. The octreotide acetate percentage was ＜90% after sodium bisulphate-containing solution was stored at room temperature under light-protected conditions for 3 days. The percentage of octreotide acetate in the pH 7.0 solution was ＜90% three days after preparation; however, in the pH 4.0 solution, it was maintained at 95% for up to 10 days. Conclusions: Our results suggest that octreotide acetate is hydrolysed in the presence of sodium bisulphate, leading to a decrease in the percentage of octreotide acetate in the solution, which can be avoided using sodium bisulphate-free dexamethasone phosphate preparations.