A multi-layer Si-based surface plasmon resonance (SPR) sensing structure, consisting of a silicon coupling prism, an intermediate SiO 2 layer, a gold film, and a sensing medium, is considered. Such structure makes possible an excitation of two angularly separated surface plasmon polariton modes over both sides of the gold film. We examine the response of the system in the case of the absorption sensing, which is simulated by the gold thickness change. Both calculations and experimental data show that the 'internal' plasmon mode over the SiO 2/gold interface appears to be at least 4-6 times more sensitive than the 'external' one over the gold/sensing medium interface, which is employed in most conventional SPR schemes. The proposed internal plasmon-based absorption sensor structure can be used for studies of optical absorption layers and for colloidal Au-enhanced SPR sensing of ultra-small (bio)-chemical agents.