Long-term performance and lifetime of vanadium redox flow batteries (VRFBs) are critical metrics in widespread implementation of this technology. One challenging issue that negatively affects these parameters is the faradaic imbalance, which is not comprehensively investigated in the literature. Faradaic imbalance is known as the shift in the average oxidation state (AOS) of the electrolyte due to side reactions. This type of imbalance requires chemical/electrochemical mitigation rather than simple electrolyte remixing. Herein, we investigate faradaic imbalance by preparing unbalanced electrolytes with differentAOS values. The performance characteristics of the flow cell utilizing electrolytes with different AOS values are reported. Based on the results of charge-discharge cycling, polarization testing, and electrochemical impedance spectroscopy measurements, faradaic imbalance is found to significantly affect discharge capacity, maximum power density, cell resistances, and efficiency values. While the ratio of discharge capacity to theoretical capacity is 83% for the ideally balanced case (AOS 3.5+), it drops to 53.4% for the AOS 3.9+ case. Additionally, there is a substantial decrease of 44% in the maximum available power density for the most unbalanced case. This noticeable performance degradation highlights the importance of faradaic imbalance as a critical factor which requires further attention especially during extended cycling. (c) The Author(s) 2019. Published by ECS.