Using high-resolution neutron diffraction measurements for Mn-rich hexagonal Mn-Fe-P-Si compounds, we show that the substitution of Mn for Fe on the 3f sites results in a linear decrease of the Fe/Mn(3f) magnetic moments, while the Mn(3g) magnetic moments remain constant. With increasing temperature, the Mn(3g) magnetic moments show almost no change, while the Fe/Mn(3f) moments decrease quickly when the transition temperature is approached. The reduction of the magnetic moments at the transition temperature and in the high-temperature range is discussed based on changes in interatomic distances and lattice parameters and high-temperature magnetic-susceptibility measurement.