  In the hadronic decay mode of a pair of W bosons, $\text{e}~+\text{e}~- \to
\text{W}~+ \text{W}~- \to \text{q}_1 \overline{\text{q}}_2 \text{q}_3
\overline{\text{q}}_4$, QCD interference effects can mix up the two colour
singlets $\text{q}_1 \overline{\text{q}}_2$ and $\text{q}_3
\overline{\text{q}}_4$, i.e. produce hadrons that cannot be uniquely assigned
to either of W$~+$ and W$~-$. We show that interference is negligible for
energetic perturbative gluon emission, and develop models to help us to
estimate the non-perturbative effects. The total contribution to the systematic
error on the W mass reconstruction may be as large as 40 MeV.
