[en] Density functional theory (DFT) calculations for deriving enthalpies of formation H for ferromagnetic MnX (X P, As, Sb, Bi) compounds were made for the two competing structures, hexagonal and orthorhombic . Standard calculations were performed by using pseudopotentials with the generalized-gradient-approximation (PBE) as exchange-correlation functional. Enhanced exchange-correlation interactions were included by making use of a so-called DFTU approach which requires as a parameter. Application of PBE potentials for all compounds and elementary phases (all-PBE) resulted in negative values of H for MnP and MnAs in both structures whereby the result for MnP agrees very well with experiment. For MnSb and MnBi the all-PBE calculation gives a positive nonbonding H disagreeing with experiment. To overcome this discrepancy for MnSb and MnBi a DFTU ansatz was employed for all compounds and elemental Mn. The values for ranging between 0.7 for MnBi and 1.4 eV for MnAs were determined by fitting the DFT results to measured data of H. As a reference for pure Mn the -Mn phase was taken with eV by which choice the experimental volume is fitted. Atomic volumes and ionicities were derived applying Bader’s concept resulting in ionicities of Mn less than . (paper)