Linear and nonlinear ultrasonic responses of contacting surfaces were studied based on the contact-pressure dependence of the interfacial stiffnesses. To this purpose, the reflection coefficients of longitudinal and transverse waves were measured for the contacting interface between polished aluminum blocks. The normal and tangential stiffnesses of the interface were then obtained as function of the contact pressure. The reflection coefficients were found to decrease, while the interfacial stiffnesses were found to increase, with the contact pressure. It was also found that the ratio of the tangential to normal stiffness as well as the nonlinearity parameters (ratio of the second harmonic to the squared fundamental amplitudes) could be used as a parameter to characterize the contact condition.