Abstract: The present invention provides a method for the synthesis of acetyl salicylic acid comprising mixing acetic anhydride and salicylic acid in approximately or exactly stoichiometric proportions and calcium oxide or zinc oxide, obtaining a yield of a mixture of acetyl salicylic acid and calcium acetate or zinc acetate with 2% maximum of free salicylic acid content. The reaction is fast, exothermic, one-pot, non-pollutant of the environment due to the fact that it doesn't require elimination of acid residues nor requires the use of any type of organic solvent (other than such solvent activity provided by the reactants themselves), and doesn't require recrystallization since it yields a dense product which can be mixed with the common excipients for acetyl salicylic acid and compressed into tablets immediately after the synthesis process.