The structure, the chemistry, and the magnetic properties of MnCoGe thin films elaborated by reactive diffusion were investigated. In situ X-ray diffraction (XRD) was used to study phase formation during thin film reaction. Mn-Co, Mn-Ge, and Co-Ge binary systems were studied before investigating phase formation during Mn-Ge-Co ternary system reaction. Three pure layers of Mn, Ge, and Co were successively deposited by magnetron sputtering on SiO2 to form a 200 nm-thick Co/Ge/Mn stack, and annealed. Six phases were observed during reaction, first following the sequential phase formation observed for the binary systems at the two Mn/Ge and Ge/Co interfaces, and ending with the formation of a single ternary compound MnCoGe at 673 K. The structure and the composition of the MnCoGe films were characterized using XRD, atomic force microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The magnetic properties of the films were studied using superconducting quantum interference device (SQUID) and ferromagnetic resonance (FMR) measurements. The obtained MnCoGe thin films are polycrystalline with the stoichiometric composition Mn:Co:Ge (1/3:1/3:1/3), and show high porosity. They are made of grains exhibiting both the Ni2In-type hexagonal structure and the TiNiSi-type orthorhombic structure.