The variation of some magnetic properties of (1) synthetic titanomaghemites and (2) oceanic basalts from DSDP site 417 D as a function of grain size and oxidation is presented. The synthesis of the initial titanomagnetite (x = 0.6) was carried out, using controlled fugacity and self-buffering techniques. Single-phase titanomaghemites were produced by heating the titanomagnetite in air at low temperatures (<350¿C). The magnetic properties studied on the synthetic titanomagnetite (grain size ~1 μm) show the following variations with increasing degrees of oxidation (0?z?0.6): (1) Curie temperature (Tc) increases, (2) saturation magnetization (Js) at room temperature increases, (3) Js measured at 77¿K decreases for z≲0.3 and then increases for 0.3<z≲0.6, (4) bulk and remanent coercivities (Hc and Hr ) increase slightly initially (z<0.2) and then decrease at both room temperature and 77¿K, (5) susceptibility (&khgr;0) decreases for z≲0.3 and then increases, (6) the magnetic viscosity acquisition and decay coefficients (Sα and Sd ) increase and (7) the median destructive fields for an anhysteretic remanent magnetization and a viscous remanent magnetization increase for z≲0.3 and then decrease. These magnetic data also suggest that the synthetic titanomaghemites are pseudo-single domained (PSD) and with increasing degrees of oxidation these PSD grains become more SD-like. The basalt samples from site 417D are divided into fine-grained pillow basalts and coarse-grained massive flows. Most of the magnetic properties measured are distinctly different between these two groups. The magnetic data indicate that the domain states of these basalts are probably PSD. Although it is difficult to separate the effects of grain size and oxidation on the magnetic properties of these basalts, it appears that grain size effects predominate. A preliminary investigation of the magnetic viscosity of the synthetic and oceanic titanomaghemites suggests that a combination of thermally activated and diffusive magnetic viscosity is responsible for their viscous behavior. Finally, we will discuss the roles of grain size and oxidation variation in oceanic basalts and their separate and combined effects on their magnetic behavior.