Scrape outer surface of orange and lemon with blade of a knife to release citrus oils. Peel orange and lemon, reserving rind. Remove and discard white membrane and seeds from pulp.
Combine orange and lemon rind, soda, and water to cover in a medium-size stainless steel saucepan. Bring to a boil. Reduce heat; simmer 5 minutes, stirring frequently. Drain. Add water to cover; bring to a boil. Boil 5 minutes, stirring frequently. Drain and cool.
Grind together orange and lemon rind, reserved orange and lemon pulp, and pears. Combine fruit and sugar in a flat bottomed kettle. Bring to a boil. Reduce heat; simmer, stirring frequently, 40 minutes or until mixture thickens. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle marmalade into hot sterilized jars, leaving 1/4- inch headspace. Cover at once with metal lids, and screw bands tight. Process marmalade in boiling-water bath 10 minutes.