We used two sets of experiments to evaluate the main and interactive effects of humic acids (HA) and anionic (AS) and nonionic (NS) surfactants on aggregate stability of two soils: a fragile, sandy loam, Typic Psammaqnent and a strong, clay, Vertic Xerochrept. For the main effects, the soils were treated at six rates, 0, 10, 50, 100, 1000, and 10000 mg/kg, with either HA, AS, or NS. A 3 x 4 factorial design, comprising three HA rates (0, 50, and 1000 mg/kg) and four AS or NS rates (0, 10, 100, and 1000 mg/kg), was used to assess the interactive effects. Changes in aggregate stability were measured at both the macro- and micro- levels by wet-sieving and clay dispersion techniques, respectively.On both soils, the percentage of water-stable aggregates increased with increasing rates of HA and NS, whereas aggregate stability was reduced progressively with increasing concentration of AS. The HA was, however, relatively more effective on the more fragile soil, whereas the converse was true for the NS. At higher NS application rates (100 mg/kg), clay dispersion decreased on the clay but increased slightly on the sandy loam soil, indicating, respectively, increased and decreased microaggregate stability. The HA and AS, however, increased clay dispersion on both soils at higher rates.The interaction of HA with NS was positively synergistic in increasing the stability of aggregates at macro- and micro- levels in both soils. The HA-AS interaction, however, reduced the stability of the sandy loam, but increased that of the clay soil at both levels of aggregation. The possible modes of action of these additives and the management implications of their effects on soil structure are emphasized.