In the present study, the effect of pH of the surrounding medium on the radical-scavenging activity of quercetin in the presence of vitamin C (ascorbic acid) was investigated. It was found that at pH 4.5-9.0 radical-scavenging activity of quercetin in the TEAC (Trolox Equivalent Antioxidant Capacity) assay is strongly reduced by ascorbic acid as a result of their interactions. Some relevant quantum-chemical calculations were performed to get some insight into the mechanism of observed antagonistic interaction between these two popular antioxidants.