Fourier transforminfrared spectrometry, visual transmission spectroscopy, and in situellipsometry have been performed on plasma beam deposited (PBD) amorphous hydrogenated silicon layers. From these measurements refractive index at infrared wavelengths and at 632.8 nm, the optical band gap and the hydrogen content of the layers have been determined. The hydrogen concentration of the layers varies between ~9 and 25 at.¿%. It was found that the refractive index decreases more with hydrogen concentration in the layer than predicted by theoretical calculations assuming tetrahedral structures. The band gap of the material remains constant at ~1.72 eV for the range of hydrogen contents measured. The resonance frequency of the SiH stretching mode (around 2000 cm-1) increases with increased hydrogen content. This is additional evidence to support the assumption that clustered SiH (SiH on voids) does not have its stretching mode near the 2100 cm-1 SiH2 peak. From the results presented it is concluded that PBD layers show behavior similar to plasma enhanced chemical vapor deposition layers with respect to the hydrogen content in the layers.