In a typical human brain 31P NMR spectrum, a small peak is often present at ~-9.7 ppm, near the α-ATP and NAD+/NADH signals. This 31P resonance, accounting for ~1/30th of the brain α-ATP signal, has been considered to be a doublet and assigned to UDPG. Here we present strong evidence to show that the -9.7 ppm signal is in fact a finely structured quartet. This finding has direct impact on current 31P NMR method for evaluation of brain redox, which requires UDPG correction. It may also have implication on our interpretation of brain energy metabolism based on 31P NMR data.