The first examples of stable, mononuclear 17-electron carbonyl complexes of Mo(III) have been synthesized, isolated, and characterized by IR and EPR spectroscopy. Oxidation of Cp*MoCl(CO)(PMe(3))(2) (1; E(1/2) = -0.48 V vs Fc/Fc(+)), Cp*MoCl(CO)(dppe) (2; E(1/2) = -0.44 V), and CpMoCl(CO)(dppe) (3; E(1/2) = -0.25 V) with Fc(+)PF(6)(-) yields [1]PF6, [2]PF6, and [3]PF6, respectively. The IR stretching vibration of the 17-electron oxidation products are 136-153 cm(-1) blue-shifted with respect to the corresponding stretching vibrations of the parent Mo(II) compounds. The room temperature EPR spectra show observable coupling to the Mo and P nuclei and indicate a trails geometry for 1(+) and a cis geometry for 2(+) and 3(+). The four-legged piano stool geometry of 2(+) with the phosphines atoms in relative cis positions has been confirmed by a single-crystal X-ray analysis.