In-situ electron spin resonance (ESR) measurements during H-2 plasma and Ar plasma treatments on hydrogenated amorphous silicon (a-Si:H) have been reported to elucidate the chemical reactions during plasma processes. From the experimental data, it is concluded that many dangling bonds (dbs) are created during the plasma process and play an essential role in chemical reactions. The difference of surface reactions between H, and Ar plasma has been clearly observed. H atoms penetrate deeply into a-Si:H network, at approximately 100 nm and create dbs, while excited Ar atoms react only with the top surface region resulting in a high db density. The difference of transient behavior between two processes is also highlighted.