Alkaline fly ash was characterized for the leaching potential of some major and minor constituents, and then added to swine manure at 10% and 20% w/v ratio. The influence of fly ash on microbial production of CO2 was studied during a 12-day experimental period. pH, ammonium- and total N, organic C, available B, available and more refractory fractions of P, K, Mg, Cu, Fe, Mn and Zn were determined on the fly ash-amended manure. On the basis of the results, it appears that the reduction in CO2 production which occurred in the amended manure was probably due to the high pH values of the manure caused by fly ash addition, rather than to an inhibition of microbial activity and respiration. A marked mobilization of inorganic P compounds of fly ash, not correlated with the gradual acidification of the mixtures during the experimental period, occurred in the amended manure, possibly as a consequence of the microbial activity. Boron was solubilized in the 20%-amended manure at an extent that might be detrimental to crops.