Chrysotile and its dimethylsilyl (DMS) and dimethylphenylsilyl (DMPS) derivatives were studied by Fourier transform infrared-photoacoustic spectroscopy. In the Si-O stretching region of chrysotile a new band was revealed at 985 cm(-1), besides absorptions at 1083, 1028, and 947 cm(-1). The Si-O stretching frequencies did not undergo major changes in the DMS derivative, but the 985- and 1028-cm(-1) peaks were undetected in DMPS due to the HCl attack on chrysotile tetrahedral sheets. Similar effects were observed in the region 900-400 cm(-1), by a decrease in intensities of the 600- and 642-cm(-1) Mg-OH libration modes in the DMPS spectrum, indicating also a HCl attack on the octahedral sheet. The Si-C band at 800 cm(-1) in the spectra of both DMS and DMPS was accompanied by minor components. DMPS showed a strong peak at 813 cm(-1) assigned to a Si-phenyl vibration. A sharp peak at 1263 cm(-1) in the DMS spectrum was ascribed to a diagnostic C-H bending mode of the dimethylsilyl groups in DMS. The complex bands around 1413 cm(-1) in DMS were attributed to CH3 deformation vibrations and that at 1466 cm(-1) in DMPS to phenyl groups. In DMPS a distinct peak at 1593 cm(-1) was attributed to a Si-phenyl vibration. In the region 3700-2500 cm(-1) absorptions at 2964, 2931, and 2907 cm(-1) in DMS were ascribed to C-H-stretching vibrations of dimethylsilyl groups, while a strong peak at 2919 cm(-1) in the DMPS spectrum was attributed to a Si-C6H5 mode.