The invention concerns a two-stage process for the manufacture of a colloid of iron oxide, and results in the dispersion of the desired iron oxide particles within a carrier fluid by a mixture of mono- and polycarboxylic acids providing excellent properties to the colloid. The process also has the advantage of preserving the physical form of the iron oxide particles, allowing any desired properties of the iron oxide core (such as crystal form or magnetism) to be maintained within the colloid.