Prepare preserving jars. Slice oranges and lemons thinly, discarding the ends and seeds. Chop very finely or processin a food processor until finely chopped but not pureed. Combine all ingredients in your preserving pot. Bring to a boil over medium-high heat. Cook rapidly, stirring constantly, until the mixture thickens, about 20-25 minutes. Test for gel stage, using thermometer or plate test.
Remove from the heat. Skim off foam if needed. Stir for 3-5 minutes to prevent floating fruit. Pour marmalade into hot sterilized jars, leaving 1/2" head space. Wipe the rims clean. Seal according to manufacturer's directions. Process the jars in a boiling water bath for 10 minutes. Makes 6 8-oz. jars.