A multi-methodic analysis was performed on 5 samples of fly ashes coming from different biomasses. The aim of the study was to evaluate their possible re-use and their dangerousness for men and environment. Optical granulometric analyses indicate that the average diameter of the studied fly ashes is around 20 µm, whereas only ~1 vol% has diameter lower that 2.5 µm. The chemical composition, investigated with electron probe microanalysis, indicates that all the samples have a prevalent Ca composition, followed by Si and Al. A large content in K and P was observed in some samples, whereas the content in potentially toxic elements is always below the Italian law thresholds. Polycyclic aromatic hydrocarbons are completely absent in all the samples coming from combustion plants, whereas they are present in the fly ashes from the gasification center. Quantitative mineralogical content, determined by Rietveld analysis of X-ray powder diffraction data, indicates that all the samples have a large amorphous content, likely enriched in Ca, and several K and P minerals, such as sylvite and apatite. The results obtained from the performed chemo-mineralogical study allowed to point out that the biomass fly ashes could be interesting materials (1) as amending in clayey soils, in substitution to lime, to stimulating pozzolanic reaction and improve their geotechnical properties, on the one hand, avoiding to mine raw materials and, on the other hand, re-cycling wastes; (2) as agricultural fertilizes made by a new and ecological source of K and P.