We studied by infrared and Raman spectroscopy the interaction of calf thymus DNA with various types of silica particles, ranging in size from 7 nm to 60 mm. Preliminary experiments with different samples showed that substantial variations can take place in the 1000-1100 cm 01 region of the attenuated total reflection (ATR) spectrum of silica, where a strong band due to a stretching vibration of the Si-O groups occurs. The position and intensity of this band were found to be dependent on several parameters, such as the size distribution of the solid particles, their proximity to the surface of the ATR crystal, and their degree of packing after sedimentation from the aqueous suspension. Changes observed in the spectra of aqueous solutions of DNA interacting with silica particles are explained by a shift of the main silica band in the mixtures. This interpretation differs from that of a previous study, where important intensity variations of the DNA bands at 1086 and 1053 cm 01 were explained by the formation of hydrogen bonds between the silanol groups of silica and the phosphate groups of DNA. Raman spectra of aqueous solutions of DNA mixed with fumed quartz particles of an average size of 0.007 mm showed but a minor change in intensity (ca. 5%) of the DNA symmetric phosphate band, which supports the conclusion reached in our infrared study.