For a thorough mechanistic understanding of reverse osmosis (RO), data on ion retention obtained by desalination of multi-ionic solutions are needed. In this paper, we show how to obtain such data under controlled laboratory conditions at any nonextreme pH. For that, we propose a simple method where we use N2 and CO2 gas control to set the composition of a gas phase in equilibrium with the feedwater solution. By increasing the CO2 partial pressure, the pH of the solution will decrease and vice versa. We applied this method of CO2 gas control to extend and validate an existing data set on ion retention of multi-ionic brackish water with 10 different ionic species, whereas conditions in the prior data set were slightly uncontrolled; in our new analysis, we performed experiments at precisely controlled pH and temperature. We run experiments at pH 6.73 and pH 7.11 and in a temperature range of T = 15−31 °C. Our results show that when pH is decreased, or temperature increased, the ion retention of most ions decreases. We also tested the influence of the Na+ to Ca2+ concentration ratio in this multi-ionic solution on ion retention at pH 6.73 and T ∼ 31 °C. We noticed that this ratio has a larger effect on ion retention for cations than for anions. We compare our data with the earlier reported data and describe similarities and differences. The improved data set will be an important tool for future development of accurate and validated RO ion transport models. Such RO models that describe desalination performance in detail are important for successful commercial application of the RO technology. We also discuss a relevant preparation method for water slightly oversaturated with barely soluble CaCO3 by solution preparation at high CO2 pressure, after which the solution is brought to the required pH by the N2 and CO2 gas control method.