Jet-cooled diatomic PtSi, produced in a laser ablation supersonic expansion source, has been spectroscopically investigated between 17 400 and 24 000 cm(-1) by resonant two-photon ionization spectroscopy. Two vibrational progressions are observed and identified as the [15.7]Omega＇=1 <-- X (1)Sigma(+) and [18.5]Omega＇=1 <-- X (1)Sigma(+) band systems. Three bands in the former system and six bands in the latter system were rotationally resolved and analyzed, leading to bond lengths of r(e)＇=2.1905(13) Angstrom and r(e)＇=2.2354(3) Angstrom for the [15.7]Omega＇=1 and [18.5]Omega＇=1 states, respectively. The Omega"=0 ground state of PtSi is assigned as a (1)Sigma(+) state, in agreement with previous work and with the assigned ground states of the isovalent NiC, PdC, PtC, and NiSi molecules. The ground state bond length of PtSi is given by r(0)"=2.0629(2) Angstrom. A Rydberg-Klein-Rees analysis of the ground and excited state potential energy curves is presented, along with a discussion of the chemical bonding and a comparison to the isoelectronic molecule, AlAu. Evidence is presented for a double bond in PtSi, as opposed to a single bond in AlAu. (C) 2000 American Institute of Physics. [S0021-9606(00)00707-8].