One class of maraging steels are strengthened by a combination of β-NiAl and η-Ni3Ti intermetallic phases which are precipitated during an aging heat treatment. To establish a meaningful structure-property relationship the precipitation strengthening effect from each phase must be isolated from the other strengthening mechanisms. To achieve this, a series of model alloys based on the Fe-12Ni alloy system were aged and characterized to determine the precipitation strengthening effect. In the course of this study, using a new approach, atom probe tomography was used to determine the spacing between precipitates and to calculate the individual strength contribution of the β and the η phase using a model describing the precipitate-dislocation interactions. It was found that the precipitation strengthening of the combined β and η phases is close to 1000 MPa and that the relative strengthening effect of each phase is sensitive to the Ti and Al concentration.