  Polarisation labelling spectroscopy technique was employed to study the
3$^{1}\Pi_{u}$ state of Cs$_2$ molecule. The main equlibrium constants are
$T_e=20684.60$cm$^{-1}$, $\omega_e=30.61$cm$^{-1}$ and $R_e=5.27$\r{A}.
Vibrational levels $v=4-35$ of the 3$^{1}\Pi_{u}$ state were found to be
subject to strong perturbations by the neighbouring electronic states. Energies
of 3094 rovibronic levels of the perturbed complex were determined.
