The short-lived isotopic system involving the decay of ¹⁸²Hf to ¹⁸²W (half life of 9 Ma) has been used to establish a time-scale for planetary assembly and core formation. The W isotope signature of lunar and martian basalts appears to be correlated to characteristics of their source. On the Moon, the small amount of excess ¹⁸²W is associated with basalts that may be derived from an ilmenite- and clinopyroxene-bearing mantle that was produced during late-stages of lunar magma ocean crystallization. On Mars, excess ¹⁸²W is associated with a depleted mantle source. Our experimental measurements for the partitioning behavior of Hf and W between silicates and melt suggest that the early fractionation of Hf from W by phases such as ilmenite, high-Ca clinopyroxene, and garnet may be significant enough to account for the generation of all or part of the radiogenic W signatures in the mantle sources for these basalts. The duration and nature of early mantle differentiation events on the Moon and Mars may be calculated by combining Hf-W fractionation with the extent of radiogenic W enrichment.