The 6(1)Pi(u) state of sodium dimer has been observed up to v = 53 in excitation spectra of the 6(1)Pi(u) <- X-1 Sigma(+)(g) system, recorded by polarisation labelling spectroscopy technique. The Dunham coefficients are derived and the potential energy curve constructed by the inverted perturbation approach method. Equilibrium constants for the 6(1)Pi(u) state of Na-2 are: T-e = 35446.06 +/- 0.04 cm(-1) (with respect to the minimum of the electronic ground state), Y-10 = 111.388 +/- 0.019 cm(-1), Y-01 = 0.112122 +/- 0.000017 cm(-1). (c) 2006 Elsevier B.V. All rights reserved.