X-ray, chemical and thermomagnetic analyses were made on ferromagnetic minerals separated from six submarine basalts of various ages, which are dredged from seamounts and ridge or drilled from sea floor. Lattice parameters, Curie temperatures and oxidation parameters are determined for these samples. From a comparison between K-Ar or Ar39-Ar40 ages and the magnetic properties, it is evident that major ferromagnetic minerals in young submarine basalts are titanomagnetites, whereas older submarine basalts contain essentially titanomaghemites. We conclude that the primary titanomagnetites have undergone maghemitization in suboceanic environment. The maghemitization process is estimated to have a time constant of a few tens of millions of years and maghemitized depth may extend to a few tens of meters in older (say older than 5X10y) oceanic crust.NRM of several maghemitized submarine basalts are subjected to a-f, pressure and low temperature demagnetization, hoping to see if any of the demagnetization techniques could discriminate a secondary magnetization which might be acquired during the maghemitization. Although the demagnetization techniques erased considerable portion of the NRMs, none of them was successful to demagnetize preferentially a remanent magnetization carried by titanomaghemites.