Measurements of the densities of the molecular and atomic ions (n(H)(+), n(H2)(+), n(H3)(+)) were carried out in hydrogen plasma with a hydrogen gas puff at discharge current of 50 A. The molecular and atomic ion currents were detected using an "omegatron" mass analyzer. The electron density and temperature were measured using a Langmuir probe and a double probe. From the measured parameters, a zero-dimensional model using the relevant rate balance equations was found to predict the observed dominant ion density.