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Repeated geodetic (GPS and InSAR) observations in the South Iceland Seismic Zone (SISZ) show transient surface deformation signals following two M6.5 earthquakes in June 2000. Previous studies concluded that the postseismic signal observed during the 2000-2005 time period was primarily due to viscoelastic relaxation in the lower crust and upper mantle, rather than afterslip. Here, we add continuous and campaign GPS observations from 2005 to 2008. Our analysis indicates that the postseismic signal is very small after 2005. We test viscoelastic models using Maxwell and standard linear solid (SLS) rheologies to explain the transient signal observed during 2000-2008. In order to explain the rapid decay in the surface velocities we observe in the longer GPS time series, our preferred viscoelastic model consists of a thick (similar to 15 km) elastic crust overlying an upper mantle with SLS rheology (a viscosity of 1-3.10(18) Pa s and relaxation strength between 0.1 and 0.25). Our preferred model differs from previous models for the postseismic relaxation in the SISZ in that we find an elastic lower crust and a more rapidly relaxing upper mantle rather than a lower crust with a Maxwell rheology and a viscosity of 1.10(19) Pa s. The June 2000 earthquakes were followed eight years later by an earthquake doublet of composite magnitude 6.3 in the western part of the SISZ. We find that the Coulomb failure stress change due to viscoelastic relaxation increased the loading of the faults that ruptured on 29 May 2008 by about 0.05 MPa. This stress increase is, however, an order of magnitude smaller than the static stress changes estimated for the June 2000 main shocks. The viscoelastic relaxation thus loaded the faults that failed in May 2008, but additional loading processes may need to be considered to fully explain the time delay between the June 2000 and the May 2008 earthquakes. (C) 2011 Elsevier B.V. All rights reserved.