The growth of the 3.3 V discharge plateau in the voltage curve of Li/Li1+xMn2-xO4 cells was followed during cycling. We believe this plateau is a result of oxygen deficiency in the sample. It is found that the 3.3 V discharge plateau grows upon cycling and while progressively charging to higher voltages, suggesting that Li1+xMn2-xO4 samples lose oxygen to the electrolyte. A correlation between the growth of this plateau and the capacity fading upon cycling is demonstrated. Samples having better capacity retention upon cycling are also more resistant to developing the 3.3 V discharge plateau. We believe that this is because well-performing samples are more resistant to oxidizing the electrolyte during cycling, especially in the charged state, Therefore the technique of following the growth of the 3.3 V plateau upon cycling is very useful in screening for stable Li1+xMn2-xO4 materials for Li-ion battery applications.