The present invention provides means to improve the chemical stability of aqueous, parenteral pharmaceutical compositions comprising a polypeptide and glycerin. Reactive aldehydes are identified in commercial glycerins, and means for reducing such are provided. Convenient means are provided to assay for reactive aldehydes in glycerin, and a strong linear correlation between the level of reactive aldehydes in glycerin and chemical stability of compositions comprising a polypeptide and glycerin is demonstrated. The invention includes aqueous compositions comprising a polypeptide and glycerin having improved chemical stability compared to compositions previously known.