An rf ion source of conventional type is shown to yield a current of 2.2 μa of negative hydrogen ions. This is accomplished by charge exchange of the positive ions emerging from the plasma with the neutral gas in the probe channel. The negative ion beam is well focused, and the performance of the source is 0.22 μa/cc at NTP/hour. The differential equations for the above charge exchange process are set down and solved, and found to be consistent with the observations to within an order of magnitude using published cross sections. The theory also predicts a sizable neutral beam (∼60 μa) of energy 2–3 kev. According to the equations and the published positive ion performance of these sources, many times larger negative hydrogen ion yields should be obtainable if required.