X-ray diffraction (XRD), transmission Mössbauer spectroscopy, and vibrating sample magnetometry were used to study the influence of precipitates on the magnetic properties of aged Fe–Cr–Mo alloys. XRD patterns and Mössbauer spectra showed that the alloys have body-centered cubic structure similar to that of α-Fe. Small amounts of precipitates were also identified with relative fraction found to be related to Mo content, aging times, and temperatures. Magnetic measurements showed that the increase in the density of precipitates contributes significantly to the increase in the magnetic hardness of the material. This mechanism is related to a process of pinning of magnetic domain walls. It was also found that saturation magnetization is affected by the Mo content in the alloy. The techniques used in this work were shown to be very useful to understand the mechanisms through which the formation of precipitates affects the magnetic properties of the alloys and may be used as complement to the usual microscopy-based techniques for this purpose.