An extended overview of the phase-mineral and chemical composition and classification of biomass ash (BA) was conducted. Some general considerations related to the composition of BA and particularly problems associated with this issue were discussed initially. Then, reference peer-reviewed data including phase-mineral composition and properties of BAs plus own investigations were used to describe and organise the BA system. It was found that BA is a complex inorganic-organic mixture with polycomponent, heterogeneous and variable composition. The phase-mineral composition of BA includes: (1) mostly inorganic matter composed of non-crystalline (amorphous) and crystalline to semi-crystalline (mineral) constituents; (2) subordinately organic matter consisting of char and organic minerals; and (3) some fluid matter comprising moisture and gas and gas-liquid inclusions associated with both inorganic and organic matter. Approximately 229 forming, major, minor or accessory phases or minerals were identified in BA. These species have primary, secondary or tertiary origin in the combustion residue and they are generated from natural (authigenic and detrital) and technogenic phases or minerals originally present in biomass. Common topics related to BA such as: terminology clarification; chemical composition; contents and concentration trends; correlations and associations; formation and behaviour; fusion temperatures; and leaching; were discussed and compared to coal ash. A general characterization of the phase-mineral composition and description of the occurrence and origin for common constituents in BA, namely: (1) silicates; (2) oxides and hydroxides; (3) sulphates (plus sulphides, sulphosalts, sulphites and thiosulphates); (4) phosphates; (5) carbonates (plus bicarbonates); (6) chlorides (plus chlorites and chlorates); (7) nitrates; (8) glass; (9) other inorganic phases; (10) organic phases; and (11) organic minerals; were also conducted and compared to coal ash. Finally, certain major associations related to the occurrence, content and origin of elements and phases were identified in the BA system and they include: (1) Si-Al-Fe-Na-Ti (mostly glass, silicates and oxyhydroxides); (2) Ca-Mg-Mn (commonly carbonates, oxyhydroxides, glass, silicates and some phosphates and sulphates); and (3) K-P-S-Cl (normally phosphates, sulphates, chlorides, glass and some silicates and carbonates). It was found that these systematic associations in BA have a key importance in both fundamental and applied aspects, namely their potential application for classification and indicator purposes connected with innovative and sustainable processing of BA. The potential utilization, technological and environmental advantages and challenges related to BA using the above classification approach are described in Part 2 of the present work. (C) 2012 Elsevier Ltd. All rights reserved.