In a heavy 2 quart saucepan, over medium heat, bring to a boil sugar, corn syrup, salt, and water. Stir until sugar is dissolved. Stir in peanuts. Set candy thermometer in place, and continue cooking. Stir frequently until temperature reaches 300 degrees F.
After that, take if off the heat; quickly stir in butter and baking soda; pour out onto cookie sheet.