Empirical flow laws for periodotite and diabase were used to model the viscous relaxation of large impact craters on Venus. The models were then compared to the depths of the three largest impact craters (≥60 km diameter) measured by Venera 15/16 radar altimetry. The maximum crater retention times predicted by models that assumed ≥0.03% wt. water in the upper mantle were inconsistent with the observed depths of these craters and a mean minimum age of 0.5 Ga. Models that assumed an upper mantle water content of ≤0.003% wt. were consistent with the observations and a maximum crater retention time of 1.0 Ga. This result was insensitive to up to 60% by volume diabase mixed into the upper mantle as inclusions at the several kilometer scale and to the presence of a basalt-eclogite or melting transition at a depth of 100 km. ¿ American Geophysical Union 1991