Uniform cobalt ferrite particles were prepared by conversion from beta-FeOOH and beta-Co(OH)(2) particles as an application of the recently developed 'gel-sol method'. A concentrated NaOH solution was added to a highly condensed suspension of beta-FeOOH and beta-Co(OH)(2), containing small amounts of FeCl3 and CoCl2, to adjust the pH to 13, and it was aged at 40 degrees C for 30 min to obtain nucleation of the cobalt ferrite. After the nucleation stage, the pH was lowered to 11 and the temperature was raised to 80 degrees C, followed by aging at this temperature for a week to grow the ferrite particles. The arrangement of pH and use of beta-FeOOH and beta-Co(OH)(2) of low solubilities as reservoirs of Fe3+ and Co2+ ions contributed to the formation of uniform particles by preventing the renucleation and coagulation of the particles during the growth stage.