Small particles of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or a mixture thereof provide a higher Limiting Oxygen Index ("LOI") to polyolefin compositions containing the small particles than do larger particles of the same substance. In many instances, improvement in one or more other properties, such as for example, processability, appearance, UL-94 rating, tensile strength, tensile modulus, and/or elongation at break is also obtained.