The concentration of hydrogen in solution near the surface of silicon exposed to an cyclotron resonance (ECR) plasma was determined by measuring the flux of hydrogen permeating to subsurface microcavities. The energy and flux of hydrogen impinging onto the surface from the plasma was also measured. A model is described which predicts the concentration of hydrogen in solution from the energy and flux of the impinging hydrogen. The measured solution concentrations were approximately 10(-9) H/Si at 600-degrees-C and approximately 10(-8) H/Si at 400-degrees-C, in fairly good agreement with the model. The absence of accumulation of immobile hydrogen near the surface indicates that lattice defects, which strongly trap hydrogen, were not produced by the ECR plasma. This study establishes a connection between the properties of the ECR plasma and the concentration of mobile hydrogen in silicon samples exposed to the plasma, which allows improved control over passivation of defects and dopants.