We have studied the magnetic birefringence in aqueous suspensions prepared from cathode deposits containing up to 5% of multi-walled carbon nanotubes and in suspensions of some other carbon materials. The magnetooptical effect observed in the suspensions of materials with nanotubes can be considered as a superposition of two contributions, one being related to the presence of graphite and graphitelike structures in suspension and the other, to the multi-walled nanotubes. The magnetic birefringence technique can be used to determine the content of multi-walled nanotubes and to obtain carbon materials enriched with nanotubes.