Gaseous elemental mercury (GEM) and <sup>222</sup>Rn, a radioactive gas of primarily terrestrial origin with a half-life of 3.8 days, have been measured simultaneously at Cape Point, South Africa, since March 2007. Between March 2007 and December 2011, altogether 191 events with high <sup>222</sup>Rn concentrations were identified. GEM correlated with <sup>222</sup>Rn in 94 of the events and was constant during almost all the remaining events without significant correlation. The average GEM / <sup>222</sup>Rn flux ratio of all events including the non-significant ones was −0.0001 with a standard error of ±0.0030 pg mBq<sup>−1</sup>. Weighted with the event duration, the average GEM / <sup>222</sup>Rn flux ratio was −0.0048 ± 0.0011 pg mBq<sup>−1</sup>. With an emission rate of 1.1 <sup>222</sup>Rn atoms cm<sup>−2</sup> s<sup>−1</sup> and a correction for the transport time, this flux ratio corresponds to a radon-calibrated flux of about −0.54 ng GEM m<sup>−2</sup> h<sup>−1</sup> with a standard error of ±0.13 ng GEM m<sup>−2</sup> h<sup>−1</sup> (<i>n</i> = 191). With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa seems to be a net mercury sink of about −1.55 ng m<sup>−2</sup> h<sup>−1</sup>. The additional contribution of an unknown but presumably significant deposition of reactive gaseous mercury would further increase this sink.