Polyester polyol compositions are disclosed. The polyol compositions, which comprise recurring units of a digested thermoplastic polyester, a glycol, and castor oil, ricinoleic acid, or a mixture of castor oil and ricinoleic acid, have hydroxyl numbers within the range of 20 to 150 mg KOH/g and average hydroxyl functionalities within the range of 2.5 to 3.5. The invention includes flexible polyurethane foams that incorporate the polyester polyols. Sustainable polyester polyols made completely or in substantial part from recycled, post-industrial, and/or biorenewable materials such as polyethylene terephthalate, glycols, and castor oil are provided. The polyols have desirable properties for formulating flexible polyurethane foams and other products.