Possible large mass effects in direct determination of the CKM elements in top decays

We discuss the possibility that mass effects, beyond phase space, may have substantial influence on the direct determination of $V_{tq}$ in decays of top quarks. In principle, our results are valid both for singly produced top and for $t {\bar t}$ pair production. The mass effect is practically irrelevant for the extraction of $V_{tb}$ from Tevatron data, but it may have implications for higher energy multi-jet top decay.