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 CH 3 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-C 6 H 5 mode.