We present an analysis of two-loop mixed QCD and electroweak
corrections to the decay of the Z boson into light quarks.
We find that the naive factorization of QCD and electroweak
corrections does not describe correctly the two-loop effects.
The nonfactorizable corrections shift the width of the Z boson
by approximately -0.55(3) MeV and increase the central value of the
strong coupling constant determined at LEP by 0.001.