We present experimental observation of the B-1 Pi state in NaCs molecule converging to the Na (3(2)S(1/2)) + Cs (6(2)P(3/2)) states of separated atoms. The two-colour polarisation labelling spectroscopy technique was applied to the B-1 Pi <- X-1 Sigma(+) band system. The data field for the B state consists of rovibrational levels with nu = 0-52 and J = 13-51, the last vibrational level located ca. 7.6 m (1) below the atomic asymptote. The data were incorporated into a fit of a potential energy curve to the level energies using the Inverted Perturbation Approach (IPA) method. The resulting potential, extending between R = 2.4 and 24.8 angstrom, is employed to partially assign the existing photoassociation spectra of NaCs. (C) 2010 Elsevier B.V. All rights reserved.