  We compare the effectiveness of various tests of three-flavor mixing in
future long-baseline neutrino oscillation experiments. We analyze a
representative case of mixing in a simplified three-flavor scheme, whose
relevant parameters are one neutrino mass-square difference, $m^2$, and two
mixing angles, $\psi$ and $\phi$. We show that an unambiguous determination of
$\psi$ and $\phi$ requires flavor-appearance tests in accelerator experiments,
as well as supplementary information from reactor experiments.
