Addition of amendments to reduce P solubility is one of the most effective in-situ technologies for remediating P contaminated soils. Abundant evidence indicates that drinking-water treatment residuals (WTRs) are effective amendments. However, little information is available concerning the stability (the longevity of WTR immobilization) when important soil parameters change. Using a modified isotopic dilution technique, coupled with a stepwise acidification treatment, we investigated the effects of aging and pH changes on the lability of P in P impacted soils treated with Al-WTR. We utilized artificially aged- and field-aged soil samples for this study. Artificial aging was achieved through thermal incubation of soil samples for 3.5 yr, and by multiple wet-and-dry cycles for 1 yr. Field samples were obtained from a one-time WTR amended plots, sampled yearly for 6.5 yr. Within pH range of 4 to 7, labile P concentrations of the WTR-amended field samples were reduced to < 46 % of the soil samples without WTR amendments and the reduction persisted for 6.5 yr. The labile P concentrations of the WTR amended soil samples aged through wet-and-dry cycles were also reduced to < 40 % of the samples without WTR amendment. At pH 3, however, there were no significant differences in the respective P lability between the WTR amended and the unamended field and the artificially aged (wet-and-dry) soil samples. After 3.5 yr of thermal incubation, labile P concentrations of WTR-amended soil samples decreased to < 30 % of the samples without WTR amendment, independent of pH. We concluded that WTR is an effective amendment to control labile P in P-impacted soils and that the WTR-immobilized P will remain fixed for a long time, independent of common soil pH values.