We report the first experimental observation of the 6(1)Sigma(+) state in the KCs molecule. Polarization labelling spectroscopy technique was applied to determine energies of about 1000 rovibrational levels in this state, most of them strongly perturbed by the neighbouring 4311 state levels. The inverted perturbation approach was used to construct the irregular potential energy curve of the 6(1)Sigma(+) state. It is compared with the recent theoretical calculations to show that Hund's coupling case (a) is proper for description of this state. (C) 2014 Elsevier B.V. All rights reserved.