  We perform photoluminescence experiments at 4K on two different transition
metal diselenide monolayers, namely MoSe2 and WSe2 in magnetic fields $B_z$ up
to 9T applied perpendicular to the sample plane. In MoSe2 monolayers the valley
polarization of the neutral and the charged exciton (trion) can be tuned by the
magnetic field, independent of the excitation laser polarization. In the
investigated WSe2 monolayer sample the evolution of the trion valley
polarization depends both on the applied magnetic field and the excitation
laser helicity, while the neutral exciton valley polarization depends only on
the latter. Remarkably we observe a reversal of the sign of the trion
polarization between WSe2 and MoSe2. For both systems we observe a clear Zeeman
splitting for the neutral exciton and the trion of about $\pm2$meV at
$B_z\mp9$T. The extracted Land\'{e}-factors for both exciton complexes in both
materials are $g\approx -4$.
