Aggregation kinetics of chiral-specific semiconducting single-walled carbon nanotubes (SWNTs) was systematically studied through time-resolved dynamic light scattering. Varied monovalent (NaCl) and divalent (CaCl(2)) electrolyte composition was used as background solution chemistry. Suwannee River humic acid (SRHA) was used to study the effects of natural organic matter on chirally separated SWNT aggregation. Increasing salt concentration and introduction of divalent cations caused aggregation of SWNT clusters by suppressing the electrostatic repulsive interaction from the oxidized surfaces. The (6,5) SWNTs, i.e., SG65, with relatively lower diameter tubes compared to (7,6), i.e., SG76, showed substantially higher stability (7- and 5-fold for NaCl and CaCl(2), respectively). The critical coagulation concentration (CCC) values were 96 and 13 mM NaCl in the case of NaCl and 2.8 and 0.6 mM CaCl(2) for SG65 and SG76, respectively. The increased tube diameter for (7,6) armchair SWNTs likely presented with higher van der Waals interaction and thus increased the aggregation propensity substantially. The presence of SRHA enhanced SWNT stability in divalent CaCl(2) environment through steric interaction from adsorbed humic molecules; however showed little or no effects for monovalent NaCl. The mechanism of aggregation-describing favorable interaction tendencies for (7,6) SWNTs-is probed through ab initio molecular modeling. The results suggest that SWNT stability can be chirality dependent in typical aquatic environment.