The solubility of aqueous electrolyte solutions is a critical property for absorption refrigeration and heat pump working fluids. In this paper, we report a systematic study on the solubility of 21 alkaline nitrate and nitrite mixtures determined by a visual polythermal method. The solubility of the mixtures (LiNO3 + NaNO3 + KNO3) + H2O with salt mass ratios of 53:19:28 and 53:05:42 and (LiNO3 + KNO3 + NaNO2) + H2O with salt mass ratios of 53:35:12 and 60:36:04 was further determined accurately by differential scanning calorimetry. Saturation temperature data of these mixtures were correlated with the total salt mass fraction using empirical polynomial equations. The mixtures of LiNO3 + NaNO3 + KNO3 (53:05:42) and LiNO3 + KNO3 + NaNO2 (53:35:12) showed the highest solubility in water.