Effect of gaseous HCl and HNO3 on the water vapor uptake by pyrogenic silica was studied at different relative humidities (RK) for pure water and different compositions of binary and ternary vapor mixtures. Experiments showed that the ability of silica to uptake water strongly depends on RH and on the type of acids and their concentration in the vapor mixtures. At low acid concentration in the binary mixtures the influence of acids is probably small. Water uptake by silica does not change monotonically with acid concentration : at first it decreases and then starts to grow. However, the presence of acids promotes water uptake, and the effect is very significant at low RH. HCl seems to be more effective acid to enhance water uptake than HNO3. In the case of ternary mixtures the adsorbed weight of water is a bit larger than that adsorbed from the binary mixtures. Acids are accumulated by silica surface, and the accumulation is larger for nitric acid.