Proterozoic mafic dykes of both tholeiitic basaltic and more magnesian compositions from four areas in Finland have been studied for their magnetic and compositional variations to explain the variation in their aeromagnetic patterns. The highly magnetic more magnesian dykes are characterized with distinctive aeromagnetic anomalies, whereas the tholeiitic dykes are weakly magnetic and associated with faint magnetic signatures. To evaluate the factors that control the primary magnetization and the effects of later geological processes on their ferrimagnetic mineralogy, petrophysical properties of the dykes were correlated with their bulk mineral compositions. The role of ferric iron was of particular interest, as those iron-bearing silicates which participate in the reactions where titanomagnetites are formed or destroyed contain ferric iron in their crystal structures. The early crystallization of hydrous mafic silicates, biotite and amphibole, versus the anhydrous mafic silicate, clinopyroxene (augite), seems to be of key importance to formation of titanomagnetites. As clinopyroxenes can accommodate only minor amounts of ferric iron compared to hydrous mafic silicates, the excess ferric iron can be consumed to form primary titanomagnetites. Therefore it is concluded that the water vapour pressure at the emplacement depth of the dykes is most relevant in terms of the primary ferrimagnetic mineralogy. The originally high total magnetization of rocks that already contain abundant ferrimagnetic minerals will not be greatly changed in secondary processes, as the volume percentage of the secondary fine-grained magnetite is generally small. In addition, the total magnetization of the rocks that are originally deficient in titanomagnetites does not increase significantly.