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In this report the results of crystal structure investigations, high magnetic field measurements and electronic structure calculations carried out for the MnFe 1−x Co x P system are presented. The crystal structure parameters were determined using the x-ray powder diffraction method. On this basis the inter-atomic distances were calculated and the magnetic couplings between magnetic atoms in MnFe 1−x Co x P are discussed. Magnetic properties of the series of compounds with x = 0.3, 0.45, 0.5, 0.525, 0.55, 0.65 and 0.7, as determined under strong magnetic field (up to 20 T), are reported. The electronic band structure calculations were performed using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). The site preference of Co and Fe atoms, located in pyramidal and tetrahedral positions, was analysed and magnetic properties of Co and Fe sublattices were calculated based on total energy computations. The sitedecomposed densities of states and the magnetic moment values were calculated in the whole alloy concentration range assuming a ferromagnetic (F) order. For MnFe 0.35 Co 0.65 P the KKR-CPA calculations were carried out assuming different types of antiferromagnetic (AF) arrangement in order to elucidate the origin of the AF-F transition. The magnetic interactions between transition metal atoms, as established from the phenomenological analysis of relating magnetic couplings and inter-atomic distances, were discussed based on the evolution of the site-decomposed density of states and the corresponding dependence of local magnetic moments on alloy composition.