100g of multi-walled carbon nanotubes (MWCNTs) tangled in balls of 388m in Sauter diameter were treated per run in a pre-industrial scale fluidized bed reactor, using ozone-based gaseous mixtures at ambient temperature. The influence of ozone concentration and of the addition of water vapour was studied, for treatment durations between 1h and 20h. The process behaviour was analyzed in terms of fluidized bed pressure drop and temperature profile. The nature and amount of the grafted oxygen based functions were analyzed, as the structural modifications created. An oxidation mechanism in two steps was evidenced, showing the grafting of hydroxyl, phenol, and ether functions in a first step and then of lactone, quinone, carbonyl, and carboxylic groups. A moderate etching of the MWCNT outer walls was observed. The amount of grafted functions and of structural defects increased with treatment duration and was highly exalted by the presence of water vapour. All the results obtained showed that the oxidation was uniform on the whole powder of the bed and from the outer part to the centre of the balls, probably thanks to the high fluidization quality maintained all along the ozone treatment.