Abstract The optical transmission of Fe 3O 4 ferrofluid emulsions in either hexanes (C 6H 14) or oleic acid (C 18H 34O 2) has been investigated as a function of concentration of Fe 3O 4 ferrofluid between 6 and 100 wt%, incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 325 Oe. The optical transmission increases with decreasing concentration of Fe 3O 4 ferrofluid emulsions from 50 to 6 wt% for samples in either hexanes or oleic acid. However, under an applied magnetic field, the optical transmission decreases for samples in hexanes, and increases for samples in oleic acid. This can be explained as being due to a different aggregation ability of the magnetic colloidal particles. In hexanes, the aggregation of magnetic particles tends to increase the area covered by the magnetic colloidal particles. In oleic acid, however, the aggregation of magnetic colloidal particles tends to form pattern of mazes and thus to reduce the area covered by the magnetic colloidal particles.