Aqueous fluoride solutions underpin much of silicon processing because they are used to strip oxide layers and for cleaning. Depending on their composition and reaction conditions they can be used to create either flat, hydrogen-terminated surfaces or nanocrystalline porous silicon. Nonetheless, confusion reigns regarding the identity and quantity of different solution species as a function of the formal concentration of HF and other additives. The literature is critically surveyed, data and results are reanalyzed, and analytical expressions are derived that allow for the calculation of solution species activities. Above 0.1 mol dm(-3), fluoride solutions are highly nonideal. It is shown that the main constituents of acidic fluoride solutions are H+, F-, HF, HF2-, and H2F3- in the range of 0-2 mol kg(-1) and even as high as 6 mol kg(-1). In these concentration ranges the derived analytical expressions exhibit excellent and approximate agreement with experimental data, respectively. The nature of the HF species that exists in solution, whether it exists as a contact ion pair or as a solvated molecule, remains in doubt. (c) 2005 The Electrochemical Society.