We find that in a quartz/clay system, P- and S-wave impedances (IP and IS, respectively) each uniquely depend on a different linear combination of the total porosity and clay content. This implies that if the pore fluid is known, we can resolve these two seismically derived impedances for porosity and clay content. Key to such interpretation is rock-physics diagnostics that provide a theoretical rock-physics model to quantitatively explain well data by relating IP and IS to porosity, clay content, and pore fluid. The well data conditioned according to this model serve as input to simultaneous impedance inversion. Poisson's or the IP/IS ratio serves as the pore-fluid identifier. We give an example of such rock-physics-based interpretation of seismically derived impedances for rock properties based on well and seismic data from an offshore oil field.