Abstract: Seismic impedance is widely recognised as a powerful lithological indicator for discriminating hydrocarbon reservoirs. Ideally, once the reflectivity series is obtained, the impedance variation can be calculated by a recursive formula. In order to mitigate the accumulative errors due to recursion and to stabilise the computation, the impedance variation should be estimated by inversion, rather than a direct calculation, incorporating any geological constraints, such as well logging and borehole seismic information, in the least-squares solution. The ray impedance concept is an extension of the conventional acoustic impedance and the elastic impedance, but has a natural advantage in seismic inversion since it is defined on a specific ray parameter. Once seismic reflectivity traces are sorted according to ray-parameter values, ray-impedance inversion may be performed on constant ray parameter sections, just like a conventional acoustic impedance inversion on a stack section.