Fly ash has increasingly been used as soil amendment. The mobility of the plant nutrient, phosphorus (P), from fly ash into plant roots is limited due to various inherent locking of P in fly ash. Similarly, the phosphatic fertilizer added to acid soils is fixed and is not easily available to plants. The present study is focused on finding some important aspects of P fractions, P fixing capacity, and P adsorption behavior in acid soil and fly ash- soil mixtures to substantiate the beneficial effect of fly ash addition to acid soil in overcoming the P fixation. Fly ash collected from the fluidized bed combustion power plant of Tata Iron and Steel Company, Jamadoba, Dhanbad, India, and acid soil from Jamdoba village was mixed at different ratios for studying the behavior of P in soil and fly ash soil mixtures. In fly ash out of 3,140 mg kg(-1) total P, only 2.89% (90.95 mg kg(-1)) was in loosely bound form, whereas in soil 54.45 mg kg(-1) P (10.8%) out of the 504 mg kg(-1) total P was in loosely bound form. Most of the P in fly ash (55.0%) was associated with Ca, while it was Fe-P (40.6%) in soil. The P adsorption maximum followed the order: fly ash (9,354 mg kg(-1)), 7:1 mix (8,850 mg kg(-1)), 3:1 mix (8,547 mg kg(-1)), soil alone (8,130 mg kg(-1)), and 1:1 mix (7,194 mg kg(-1)). The supply parameter calculated from the equilibrium phosphorus concentrations and adsorption data showed that the supply parameter increased with fly ash addition. Individually, P fixing capacity of the fly ash and soil was 75.6 and 65.68%, respectively, when both were mixed (1:1), the P fixing capacity decreased to 52.94% due to the synergistic interaction between soil and fly ash. Thus, though fly ash and acid soil is having difficulty with P availability, mixing enhanced the availability of P synergistically.