Once it has cooled completely, the syrup can be stored in an airtight container in the refrigerator for up to one month.
In a medium saucepan, combine sugar and the water over medium-high heat. Cook, stirring, until the sugar dissolves. Continue cooking, without stirring, until mixture reaches a boil. Reduce heat to a simmer, and cook 5 minutes more.
Remove pan from heat, and stir in rum. Let cool to room temperature.