What you use to steam clean a couch depends on the type of machine you use. Most home and rental steam cleaners come with products recommended by the manufacturer. Use a product specifically formulated for cleaning upholstery rather than carpet and follow the manufacturer's directions carefully. If you have a true steam cleaner that has a hot water chamber that produces steam, use only water. The water is hot enough to remove dirt and grime and kill bacteria without additional cleaners.
Look for fabric labels under the cushions to provide direction on appropriate cleaning products. A "W" on the label indicates that water-based cleaners and mild detergents are safe for the fabric. Most commercial upholstery cleaners fall into this category. An "S" indicates that you must use a solvent cleaner on this upholstery. Do not use the solvent in a steam cleaner, but spot clean the upholstery instead. Use sparingly in a well-ventilated room, or hire a professional. An "X" means that the fabric can not be steam cleaned. Vacuum or brush the couch to clean it, or hire a professional.
If the couch lacks care labels, consider the type of fabric when determining how to steam clean your couch. Plain-weave linens, cottons, pile fabrics and rayon usually respond well to steam cleaning with a mild upholstery shampoo. Hire a professional to clean silk, jacquards or wool couches.
Always test any steam-cleaning products in an inconspicuous place. The two most common problems you'll encounter are dyes that bleed and fabrics that shrink. The heat generated by true steam cleaning can shrink fabrics so use these machines carefully. When in doubt, consult a professional.