Solid acid catalysts have been used as important functional materials for petroleum refinery industry and for production of chemicals. Since solid acids are enviromentally friendly with respect to less corrosiveness, safty, less waste, and ease of sepration and recovery, replacement of liquid acids with solid acids is desirable in the chemical industry. In this investigation we used silica sulfuric acid as a solid acid for some functional group interconversions. Initially, synthesis of this acid was optimized so that its capacity increased from 0.0026 mol/gr to 0.0043 mol/gr. Using NH 4 NO 3 /SSA, some aromatic compounds were nitrated to the corresponding nitro arenes. Some aryl sulfunic acids were prepared in the presence of silica sulfuric acid. Benzylic and allylic alcohols were converted to the corresponding iodides using KI/SSA system. Aldehydes and ketones were converted to the corresponding 1,3-dithiolanes in the presence of 1,2-ethanedithiol and catalytic amount of silica sulfuric acid. These reactions were carried out under solvent-free conditions. Deprotection of S , S -acetal compounds to their corresponding carbonyl compounds were carried out using NaNO 3 /SSA and wet-SiO 2 at room temperature. Aldehydes were converted to the corresponding acylals in the presence of acetic anhydride and catalytic amount of silica sulfuric acid. These reactions were carried out under solvent-free conditions. Direct sulfonylation of aromatic rings were carried out with sulfonic acids using P 2 O 5 /Al 2 O 3 . Friedel-crafts acylation of aromatic compounds were carried out using P 2 O 5 /Al 2 O 3 in the presenc of carbocxylic acids.