A Knudsen effusion technique combined with mass-spectrometric analysis of the evaporated products has been used to investigate the gaseous phase and to determine the thermodynamic properties of {xFe+yMn+(1-x-y)Si}(I) in the temperature range 1435 K to 1823 K and of {xFe+yMn+(1-x-y)P}(I) in the temperature range 1302 K to 1701 K. Concentrations of the components varied within the mole-fraction ranges x, 0 to 1; y, 0 to 0.6; and (1-x-y), 0 to 1 in the first system, and x, 0.205 to 0.738; y, 0.22 to 0.695; and (1-x-y), 0.152 to 0.365 in the second. The isoactivity lines of silicon and phosphorus are close to the secants connecting the Fe-Si and Mn-Si, Fe-P and Mn-P binary sides. Iron and manganese activities decrease regularly as the configuration point runs from {xFe+(1-x)Mn}(I), towards {xFe+(1-x)Si}(I) or {yMn+(1-y)Si}(I), and towards {xFe+(1-x)P}(I) or {yMn+(1-y)P}(I). The thermodynamic properties have been described with the associated-solution model assuming that binary and ternary complexes exist in the melt.