The electronic states of gaseous diatomic NiSi have been investigated using the combined techniques of resonant two-photon ionization spectroscopy, dispersed fluorescence spectroscopy, and density functional computations. A single electronic band system, designated as the [18.0]1<--X (1)&USigma;(+) system, has been found in the 17 500-19 500 cm(-1) range, and three bands of this system have been rotationally resolved and analyzed. To shorter wavelengths, the spectrum becomes much more congested and intense, and four bands in this region have been rotationally resolved and analyzed as well. A dispersed fluorescence investigation has allowed the measurement of 17 vibrational levels of the ground state. Through this work, the ground state of (NiSi)-Ni-58-Si-28 is demonstrated to have (1)&USigma;(+) symmetry, with r(0)=2.0316(4) &ANGS;, ω(e)=467.43(30) cm(-1), and ω(e)x(e)=2.046(21) cm(-1). These results are in good agreement with the results of density functional computations performed on the ground state. Comparisons to AlCu and CuSi demonstrate that NiSi has a double bond, while these related molecules are best considered as having a single σ-bond. (C) 2003 American Institute of Physics.