Twelve multiwall carbon nanotubes (MWCNTs) with nominally identical diameters and surface chemistry but having different average lengths in the range of 820-2160 nm, different tapped bulk densities in the range of 53143 kg/m(3) and different powder sizes in the range of 0.425-3.3 mm were analyzed 'by thermogravimetric analysis (TGA). This work shows that at heating rates normally used in TGA measurements, tapped bulk density affects the thermal degradation of CNTs. In fact, a linear correlation between CNT tapped bulk density and the ending oxidation temperature was found for a given family of CNTs with similar characteristics. Further, with different sieved particle sizes, the ending oxidation temperature did not follow the linear correlation with the tapped bulk density, indicating that the density of the individual particles also plays a role. To prove that these effects originate from mass transfer limitations, we show that at much lower heating rates the curves from various densities almost merge together and also that by compressing (i.e. increasing the density) the MWCNT powders the degradation behavior can be modified. In the literature, contradictory reports have been presented on the influence of nanotube length on the degradation profile and we believe that these contradictions are due to not considering this mass transfer effect. Additionally, since increasing the diameter and number of walls likely increases the tapped bulk density, the reported increase in thermal stability as the diameter and number of walls are increased may be also related to a mass transfer effect.