Studies have been carried out on the microwave absorption properties of the title composites. The dielectric loss of each material correlated well with its microwave absorption properties. However, these properties are not affected significantly by whether the kernel is octahedral or hollow spherical. Samples with variable amounts of polyaniline were synthesized and it was found that the maximum reflection loss occurs at around 50 % of polyaniline for both octahedral and micro-spherical Fe3O4/polyaniline composites showing that the amount of interface between Fe3O4 and polyaniline plays a key part in enhancing the absorbance although other effects are also important. Hollow spherical Fe3O4 was synthesized using ammonium acetate. However, it was noted that when sodium acetate was used, fewer cracks appeared in the material. The presence of cracks is evidence for the hollow nature of spherical Fe3O4. A clad growth mechanism is proposed for the formation of the composite material.