Total gaseous mercury (TGM) has been measured at the WMO Global Atmosphere Watch (GAW) station at Cape Point, South Africa, since September 1995, representing the only long term TGM measurement in the Southern Hemisphere (SH) so far. Annual medians suggest a small but significant decrease of TGM concentrations from 1.29 ng m−3 in 1996 to 1.19 ng m−3 in 2004. Background TGM concentrations at Cape Point varied seasonally, with a summer maximum and a winter minimum. Comparison with the seasonal variation of CO concentrations in both hemispheres calls into question the influence of mercury sink by the Hg0 + OH reaction. If confirmed at other sites in the SH, the observed TGM seasonal variation may pose an important constraint on the global models of atmospheric mercury.