This article exposes an in‐depth analysis of the chemical interaction between Cu(In,Ga)S2 thin‐film and GaP/Si(001) pseudo‐substrate, in the framework of the development of high‐efficiency tandem solar cells on Si. A rich Cu‐Ga‐P‐S chemistry at the Cu(In,Ga)S2/GaP interface is uncovered, in the case of Cu(In,Ga)S2 films grown in Cu‐excess conditions. The involved chemical reactions are studied and modeled by breaking the system down to its binary constituents, such as Cu2−xS and GaP and by characterizing the reaction products. A chemical model is proposed to explain the chemical fluctuations observed at Cu(In,Ga)S2/GaP interface. It is found that Cu‐rich Cu(In,Ga)S2 decomposes the underlying GaP film. This knowledge of Cu(In,Ga)S2/GaP interface chemical reactions can benefit the future development of two‐terminal monolithic Cu(In,Ga)S2/Si tandem solar cells.