Double-walled carbon nanotubes (DWNTs) may be interesting in many applications since the outer wall would provide an interface with the rest of the system, without modifying the inner wall. CNT-Fe/Fe3C-Al2O3 composite powders containing carbon nanotubes (CNTs) (65% of which are DWNTs) are prepared by reduction of an oxide solid solution in a H2-CH4 gas mixture. The powders and CNTs are studied by both local and macroscopical techniques. The influence of the reducing atmosphere composition and of the dwell time at 1050 degrees C is studied. There is a 6-fold increase in CNT content upon the increase in the CH4 content from 3 to 30 mol.%, but the formation of undesirable carbon nanofibers can also be promoted. A CH4 content of 12-18 mol.% is adapted for the particular iron content in these powders. Increasing the dwell time at 1050 degrees C results in the formation of CNTs with more walls.