Phosphorus (P) loss from soils can impair surface water quality. A study was conducted to test the efficacy of fly-ash to decrease phosphorus loss in 11 grassland soils. A preliminary toxicity and leaching experiment indicated that heavy metal concentrations (As, Cd, Pb, Se) in leachate and pasture from a soil treated with ash up to 50 mg/kg were not different from the control (unamended soil). Heavy metal concentrations in the ash were generally below limits for maximum concentrations in soil. Following incubation of fly ash at a rate of 20 mg/kg for 3 months with 11 grassland soils packed into boxes, overland flow was generated by simulated rainfall on each boxed soil. Analysis of overland flow indicated that in 2 semi-arid soils, P loss decreased due to decreased particulate P (PP) loss and low organic C concentration (<20 g/kg) that facilitated soil dispersion and slaking and increased soil strength. However, in 4 other soils (including 3 volcanic-ash soils with organic C >70 g/kg), P loss increased due to increased soil pH from 6 to 7 where P is most soluble. In all soils, despite an increase in P in recalcitrant soil P fractions, increased soil pH stimulated soil C and P mineralisation (decreased organic C by, on average, 4.1 g/kg), decreased soil organic P, and increased inorganic P in labile fractions. It is concluded that the application of fly-ash from this source should not be used as an amendment to decrease P loss in pastures where soil pH is commonly <6.0, but could provide useful both as a supplement to lime and in mitigating P loss in cropping soils.