The present report describes not only the effects of tannic acid (TA; belonging to hydrolyzable tannins) and its related compounds upon the infectivity of Chikungunya virus (CHIKV) but also the mechanism involved in this phenomenon. Our data show that TA inactivates CHIKV in vitro. Since the inactivating effect turned out to be pH-dependent and was suppressed by bovine serum albumin, it is most probable that the virus-inactivating capacity of TA is attributable to its preferential binding to proteins of virus particles. Examination on the virus-inactivating capacities of some TA-related compounds and comparison of their structures indicated that the active site of TA and its analogues might be the phenolic hydroxyl groups in their molecules. It seems that the active groups interact with the proteins of virus particles, resulting in a reduction or loss of viral infectivity. Discussion is made on the specificity of the actions of tannins and the possibility of application thereof to chemicals which are useful to investigate the nature and properties of viral proteins.