A Swedish research program concerning the technical and ecological effects of wood fuel ash recycling to forest biotopes has been on-going for several years. Within this program, a study of the stabilisation of ash by the so-called self-hardening process was carried out. Chemical reactions occurring in the ash materials on water addition and during solidification were identified based on results from chemical analysis, X-ray diffraction, FTIR spectroscopic and scanning electron microscopy analysis. The stability and importance of the individual reaction products are discussed, Some results of practical importance are: alkali metals are to a large extent present as salts dissolved in the pore solution, i.e. the hardened ash should be stored in a way that protects it from rainfall; the dominant wood ash element calcium Forms Ca(OH)(2) and subsequently CaCO3 during hardening and storage; ettringite formation binds sulphur and aluminium and contributes to the stability of the ash structure; and phosphorous is bound in apatite and other compounds with low solubilities.