Magnetic phase diagrams of Fe 2−ϵP (ϵ ⩽ 0.011) and (Fe 1− x Mn x ) 2P( x ⩽ 0.025) are built on the basis of measurement of the susceptibility temperature dependence in weak magnetic fields. The measurements were done on samples of single crystals at H ∥ c ( c is the direction of easy magnetization of a hexagonal crystal). Both the deviation from the stoichiometric composition of Fe 2P resulting in the appearance of vacancies, and substitution of iron by manganese give rise to evolution of magnetic phase transformations similar to the one observed in Fe 2P under pressure. Parameters of the critical points on phase diagrams T − ϵ and T − x, which limit the metamagnetic region, are found. The magnetization processes in fields up to 40 kOe are considered for samples with realization of the metamagnetic state in a wide temperature interval. The peculiarities of the obtained diagrams are explained by the prevailing role of magnetic vacancies in the magnetic phase formation in iron phosphide. An estimation has been done of the relative change of values of competing exchange interactions inside and between the magnetic sublattices formed by nonequivalent iron atoms when the parameters ϵ and x were increased.