The basicity and nature of basic species in nitrogen containing carbon nanotubes (NCNT) prepared under different conditions were investigated by acid-base titrations. Proton uptake curves were derived from the titration data and were used to establish the basicity (pK(a)) ranges of nitrogen species present in NCNT. Based on the evolution of the titration curves upon acid addition the NCNT were divided into three classes: (I) in which pyridinic type N with a pK(a) of 7-9 were predominantly present: these pyridinic groups had some buffering capacity in a pH range of 5-7; (II) where amine type N with a pK(a) of 7-9 were the major species; these groups showed only minor buffering capacity at pH 8-9; (III) NCNT with low basicity, i.e. most groups possessed a pK(a) < 7 with insignificant buffering capacity. All NCNT could be regarded as being composed of organic nitrogen containing building blocks having different pK(a) values. (C) 2009 Elsevier B.V. All rights reserved.