Leather goods are made from animal hides. Because of this, they contain tiny pores that can absorb odors in your home. If your couch is located in a place where it is exposed to kitchen odors, pet odors or cigarette smoke, it can become quite smelly over time. There are commercial products that are designed to deodorize leather furniture without damaging the finish. There are also a number of homemade remedies that you can attempt.
Apply a pH balanced leather cleaner to your couch, following the directions on the bottle. These products are designed to remove dirt and bacteria from the leather without damaging the surface, and they usually contain glycerin for adding softness to the leather.
Treat the surface with saddle soap. You can buy it from a horse and tack store, or make your own using the directions in the "Resources" section. Apply the saddle soap with a wet flannel cloth, working it into the surface with a circular motion. Polish until dry with another soft flannel cloth.
Spray white vinegar on the surface of the couch, which will kill odor-causing bacteria. Wipe away the excess with a clean cloth, and work a thin layer of oil into the surface to moisturize. Orange tree oil works well and has a fresh scent. Olive oil, neatsfoot oil and linseed oil also work well.