We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, X 1Σ+, 0000〉, J = 0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment (〈cos22D〉 = 0.84) and of state purity (>92%) for a state-selected, deflected beam compared with an undeflected beam.