Interfacial properties and relaxation dynamics of photoexcited carriers in In 0 . 5 3 Ga 0 . 4 7 As/InP multiple quantum wells (MQWs) have been investigated by means of cross-sectional scanning tunneling spectroscopy and optical spectroscopy methods (luminescence, absorption and pump-probe experiments). The MQW structure consists of 125 periods of 10-nm-wide well layers and 40-nm-wide barrier layers on an InP(001) substrate. The observed interfacial roughness of the InGaAs-on-InP is 1-2 monolayers (ML), while that of the InP-on-InGaAs is 3-4ML. The Stokes shift observed in luminescence and absorption spectra at 77K corresponds well to the well-width distribution observed by the cross-sectional STM. Differential absorption spectra measured by pump-probe spectroscopy show that relaxation of hot carriers in conduction and valence bands followed by exciton formation takes place in 17-30ps depending on the excitation photon energy. The excitons formed at the band bottom are localized at thicker areas within a quantum well in ~500ps.