Admittance spectroscopy is used to study hole states in Si0.7–yGe0.3Sny/Si quantum wells in the tin content range y = 0.04–0.1. It is found that the hole binding energy increases with tin content. The hole size-quantization energies in structures containing a pseudomorphic Si0.7–yGe0.3Sny layer in the Si matrix are determined using the 6-band kp method. The valence-band offset at the Si0.7–yGe0.3Sny heterointerface is determined by combining the numerical calculation results and experimental data. It is found that the dependence of the experimental values of the valence-band offsets between pseudomorphic Si0.7–yGe0.3Sny layers and Si on the tin content is described by the expression ΔEV exp = (0.21 ± 0.01) + (3.35 ± 7.8 × 10–4)y eV.