Polarization labelling spectroscopy is applied to study the previously unobserved 5(1)Sigma(+) state of the NaK molecule. Molecular constants and a Rydberg-Klein-Rees (RKR) potential energy curve based on these constants are determined. The values of T-e, omega(e) and R-e are 23530.62(6) cm(-1), 115.60(6) cm(-1) and 4.307 Angstrom, respectively. (C) 2002 Published by Elsevier Science B.V.