A bstractA series of experimental studies has been made on the e ect of the dilution of silane with hydrogen on optical properties of hydrogenated amorphous silicon ® lms ( a-Si : H) prepared by plasma deposition as f unctions of the gas-volume ratio g SiH 4 / SiH 4 H 2 and the substrate temperature T s . The e ect of hydrogen dilution has been discussed in terms of the obtained values of the deposition rate R d , the optical gap E g , the Urbach energy E u , the def ect density N d , the hydrogen content C H and the ref ractive index n 0 and their correlations between them and the hydrogen-bonding con® guration estimated f rom infrared absorption spectra. Its e ect strongly depends on T s and decreases N d and E u from g 100%to about 20% . The e ect f or reducing them is discussed in terms of the surf ace-limited optimal growth model containing the hydrogen dilution e ect. On decreasing g from about 10% , both N d and E u increase in spite of the decrease in R d . This result is due to the onset of the f ormation of microcrystalline phase and is discussed in terms of the`selective etching' model. An a-Si : H ® lm with E u less than 50 meV is prepared at our optimal growth conditions: g 20%and T s 200 ë C.