The colloidal stabilities of ten carbon nanotubes (CNTs) having varying physico-chemical properties were compared in long-term experiments. The presence of Suwannee River Humic Acid (SRHA) increased the fraction of CNTs in the supernatants (4-88% for the various CNT types) after addition in pre-dispersed form and 20 days of shaking and 5 days of settling. These suspensions were monomodal, containing individually suspended CNTs with highly negative surface charges. Calcium (2 mM) removed most of the CNT types from the supernatant, due to CNT-agglomerate formation initiated by reduction in surface charge. The amount of SRHA adsorbed to the different CNT types did not correlate (r(2) < 0.1) with the percentage of CNTs remaining in suspension. Multiple linear regression analysis revealed that the oxygen content and the diameter of the CNTs significantly influenced the percentage of stabilized CNTs, resulting in an increased fraction of functionalized and large-diameter CNTs that remained in suspension. (c) 2012 Elsevier Ltd. All rights reserved.