I selected these swim bottoms specifically because they were advertised as being made of Polyester, which is supposed to last longer and be more resistant to chlorine. When they came, the tag says they are 80% nylon and 20% spandex. Since there weren't a whole lot of other options out there, I choose to keep them. The material is thick and seems durable, but we will see. They fit my daughter well. We purchased a size 5 and my daughter is just finishing her size 5's and moving into 6's. They do have great coverage in the rear and sit high on the waist.