 3-Aminopropylphosphonic acid (3APPA) and sodium 3-phosphonopropionate (3PPA-Na) are investigated as organophosphorus compound additives in a positive electrolyte of a vanadium redox flow battery (VRFB) to improve its thermal stability. The precipitation-inhibiting effects of 2 M vanadium electrolytes, including the above additives, are studied by UV–Vis spectroscopy, Raman spectrophotometry, cyclic voltammetry (CV), and charge–discharge cycling test in the redox flow battery. The UV–Vis spectroscopy and Raman spectrophotometry show that no new substances formed in both the V(IV) and V(V) electrolytes with 3APPA and 3PPA-Na additives. Cyclic voltammetry (CV) shows the electrochemical activity and reversibility of the electrolytes with and without the additives. In the flow cell, the discharge capacity retention values in the positive electrolyte after 100 cycles at 45 °C are 56.7, 64.7, and 48.4% in 3PPA-Na, 3APPA, and without the additives, respectively. These results indicate that the 3APPA additive increases the discharge capacity retention to more than 16% compared to the electrolyte without the additive due to the retardation of precipitation at the high temperature.