A new empirical method for the derivation of average HO radical concentrations is presented. The method is based on estimation of CO lifetime through the relative variability of CO and Rn measurements, and hydrocarbon measurements are used to independently determine the relative effects of chemistry and dynamics. Data from Amsterdam Island (37 ¿S, 77 ¿E), a remote site in the southern Indian Ocean, are used to calculate annual and daily HO levels for 1997 and 1998. A seasonal variation in calculated daily HO, consistent with seasonally changing photolysis rates, with maxima in summer and minima in winter is also derived which is comparable but slightly lower than the most recent zonal mean HO estimates of Spivakovsky et al. <2000>. The calculated annual HO shows a decrease from 2.7¿105 molecule cm-3 in 1997 to 0.8¿105 molecule cm-3 in 1998, possibly as a result of El Ni¿o related meteorological changes. The empirically calculated HO correlates with temperature and relative humidity measured at the island but anticorrelates strongly with CO and to a lesser extent with O3. The limitations and improvements to this method are discussed. The method has potential for long-term monitoring of HO changes over areas upwind from remote sites. ¿ 2001 American Geophysical Union