Shredded rubber tires were evaluated as a bulking agent for composting waste-water sludges. Three sludges (raw primary, anaerobically digested, and secondary biological), two amendments (sawdust and recycled compost), three sizes of rubber chips (1.27 to 2.54 cm, 2.54 to 5.08 cm, and greater than 5.08 cm), and three shredded-rubber-chip-to-sludge mix ratios (1:1, 2:1, and 3:1) were evaluated. The small rubber chips, the 2:1 mix ratio, and the sawdust amendment were optimums. Tests with raw primary sludge, shredded rubber, and no amendments resulted undesirable odors and handling difficulties. A high moisture content and low C/N ratio led to sludge balling, anaerobic conditions, and the production of ammonia gas. Tests with all three sludges and recycled compost produced similar results. When amended with sawdust, all of the sludges were effectively composted using shredded rubber. All of the sludges required the moisture absorbancy and supplemental carbon that the sawdust provided. Heavy metal levels during composting with raw primary sludge and rubber chips increased due to the concentrating effect as organic matter decomposed. However, the levels were not high enough to limit the use of shredded rubber in the composting of the sludge.