A simple photoionization technique for enriching a particular isotope of mercury in a naturally-occurring mixture employing an isotopically pure radiation source separately optimized for 253.7 nm. and 404.7 nm. emission, respectively. A vessel is provided for containing the naturally occurring mercury. This vessel may be part of either a flow or closed system. The radiation source brings the isotope from the 6.sup.1 S.sub.0 ground state to the 6.sup.3 P.sub.1 state wherein the nitrogen gas deactivates the isotope to bring the isotope to the long-lived 6.sup.3 P.sub.0 state. Thereafter, the source optimized at 404.7 nm. brings the isotope to the 7.sup.3 S.sub.1 state. At this level the atom may be easily photoionized either by a laser or a powerful visible incoherent radiation source.