The apparent adiabatic compressibility and the apparent molar volume of decyltrimethylammonium bromide (C(10)TAB) at 25 degrees C were obtained from the experimental data of ultrasound velocity and density of its aqueous solutions at the concentration above and below CMC under the applied pressure ranging from 0 to 40 MPa by using the apparatus developed for this study. The application of pressure promoted hydrophobic hydration and decreased the micellar volume. It was found that isothermal compressibility of the monomeric state of C(10)TAB derived from density data was appreciably larger than the estimated value based on the sound velocity data. It is supposed that the rearrangement of hydrated water could not follow the rapid periodic change of pressure by ultrasound and led to the lower compressibility of C(10)TAB.