Many salts, such as calcium sulfate (gypsum) and barium sulfate (barite), are found prevalently in oil well lines and water treatment systems as well as on ocean vessel shells. These deposits are costly to oil refineries, water treatment plants, shipping companies, and the navy, as they cause operational issues and are difficult to remove. The presence of these deposits on ocean vessels also increases pollution due to increased drag caused by the deposits. Previous studies investigated the effect of organic molecules as inhibitors of barium sulfate crystal growth. However, few researchers have studied the growth inhibition of calcium sulfate crystals. We have found organic compounds that inhibit the crystal growth of both calcium and barium sulfate. We have synthesized a group of crystal growth inhibitors from simple diamines based upon previous barium sulfate research using the Mannich Reaction. Here we report the synthesis, purification, and analysis of several linear and one cyclic phosphonic acids. We also report the ability of these molecules to inhibit the growth of calcium sulfate crystals in supersaturated solutions.