SummaryThe effect of two hydroxides (NaOH and KOH) and two chlorides (NaCl and KCl) on aggregate stability and saturated hydraulic conductivity (Ks) was studied in three New Zealand soils using concentrations (0.003, 0.03, and 0.3m) typical of the range ofpH found in strongly alkaline industrial liquid wastes. The different solutions were used as pretreatments prior to aggregate stability measurements and as influent solutions forKsmeasurements.The concentration of hydroxide appeared to be the most important factor affecting aggregate stability andKSin all soils. Aggregate stability andKsdecreased with increasing hydroxide concentration, but were generally unaffected by chloride solutions. RelativeKsdecreased to100% with chloride solutions, but decreased substantially when distilled water replaced the chloride solutions. Differences in aggregate stability andKsdue to the cation present (either Na+or K+) appeared to be very small. A surface crust formed in the upper 1 cm of the soils leached with hydroxide solutions. This surface crust had substantially less organic carbon than the upper 1 cm of soil from the cores leached with chloride.A two‐stage process explains the decrease inkswhen strongly alkaline solutions are applied to soil. First, organic matter dissolution decreases aggregate stability, with the rate of organic matter dissolution depending on hydroxide concentration; and second, increased repulsion of soil particles (due to increasedpH) causes movement of dislodged particles into pore spaces, resulting in decreasedK