Green tea catechins (GTCs) as a mixture of (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC), and (-)-epigallocatechin gallate (EGCG) possess a variety of biological activities. We have previously studied the stability of GTCs either as a mixture or as individual epicatechin derivatives in various pH, demonstrating that GTCs as a mixture in alkaline solutions were extremely unstable and degraded almost completely in a few minutes, whereas in acidic solutions (pH<4) they were very stable. For the pH ranging from 4 to 7, the stability of GTCs was inversely associated with the pH value of the incubation solutions. The present study examined the effect of ascorbic acid and citric acid on the stability of GTCs incubated in sodium phosphate buffer (pH =7.42). Ascorbic acid added to the incubation mixture significantly increased the stability of GTCs whereas citric acid exhibited no effect. Four epicatechin derivatives examined demonstrated varying stability, with EGCG and EGC being equally instable and EC and ECG being relatively stable. The addition of ascorbic acid significantly increased the stability of all four derivatives, particularly EGC and EGCG. The present results, although not directly transferable to in vivo conditions, may suggest that the presence of ascorbic acid may stabilize the GTCs in the intestine where the pH is neutral or alkaline before absorption.