We have studied the effect of hydrogen-plasma exposure on GaAs-on-Si solar cell structures using electron cyclotron resonance (ECR). An increase in photoluminescence (PL) intensity near the GaAs/Si interface is found after the H-plasma exposure and 400 degrees C annealing, indicating passivation of threading dislocations by diffused hydrogen atoms. Secondary ion mass spectroscopy depth profiles of hydrogen concentration [H] for the ECR-plasma exposed solar cell structures have shown that the [H] near the GaAs/Si interface increases after the 400 degrees C annealing, which is in good agreement with the PL result. On the other hand, it is found that the [H] near the surface simply decreases with prolonged annealing time while the [H] near the GaAs/Si interface remains unchanged, which is favorable to the improvement of the conversion efficiency.