The pure rotational transitions of (NCl)-Cl-35 in the second electronically excited state, b(1) Sigma(+), were detected together with those of the isotopic species (NCl)-Cl-37 by microwave spectroscopy. The NCl radical in the b(1) Sigma(+) state was generated by de-glow discharge of a N-2/Cl-2 mixture. The rotational constant, centrifugal distortion constant, and quadrupole coupling constants of the Cl nucleus were determined. The observation of rotational transitions in the vibrationally excited upsilon = 1 state made it possible to derive equilibrium structural parameters. A discussion on the internuclear potential of the three low-lying electronic states of NCl is given.