In presence of liquid metal, a metal alloy, ductile in air, can be susceptible to liquid metal embrittlement (LME), which results in a loss of ductility that can cause brittle failure. LME sensitivity depends on different conditions (materials, liquid metal chemistry, strain rate, temperature, microstructure). Moreover, the mechanisms leading to LME are still poorly understood. Indeed, different models exist but their validity is only verified for very specific couples of liquid metal / metallic alloy without observations at microscopic and nanoscopic scales to explain them. In order to advance on the understanding and the prediction of LME phenomena, we have studied in detail the LME sensitivity of the copper and α-brasses in the presence of the liquid In-Ga eutectic (EGaIn), not only the conditions of LME sensitivity, but also the study of mechanisms at macroscopic and microscopic scales through in-situ observations, as well as the modeling of phenomena. The interface between the copper and the α-brasses and the liquid metal has been studied because the interface is a key point in the LME occurrence. When the EGaIn enters in contact with the copper or the α-brasses, there is the formation of the CuGa2 intermetallic which forms rapidly to hundreds of nm in tens of seconds. The intermetallic forms via the dissolution of Cu into the EGaIn and then the reaction between the dissolved Cu and the Ga. Zn also dissolves into the EGaIn but does not participate in any reaction. Despite the presence of the intermetallic, the contact angle which represents the wetting of the solid material by the liquid metal is inferior to 50° for the copper and the studied brasses. It decreases with the increasing of the Zn content in the brass. The LME susceptibility of copper and brasses containing 15, 20, 25 and 30% depends on the microstructure, the hardness, the zinc composition and the strain rate. The most important factor seems to be the composition of the copper alloy, and therefore through the plasticity mechanisms of the solid metallic material. In the cases leading to LME sensitivity, for macroscopic tests, the fracture is ductile and then brittle, which is explained by not only the formation of the intermetallic CuGa2, but also a plastic deformation necessary to initiate LME. The brittle fracture is intergranular. Tests with in situ observations with SEM (Scanning Electron Microscopy) or TEM (Transmission Electron Microscopy) showed a brittle fracture localized at the interface of the microstructure after plastic deformation of the 30%Zn-brass. Correlation between bending micro-tests and simulation by finite elements made possible to evaluate a toughness in the presence of liquid metal for the 30%Zn-brass. The value appears consistent with that obtained from simulation at the atomic scale. This work was funded by the ANR through the ANR GauguIn project (N° ANR-18-CE08-0009-01).