The present invention provides an improved process for making diphosphaspiroundecanes and product-by-process, wherein the process comprises reacting a hydroxyl-substituted organic compound, phosphorous trichloride and pentaerythritol to form an organic-substituted diphosphaspiroundecane such that formation of said organic-substituted diphosphaspiroundecane takes place in the presence of a tri-n-alkylamine having n-alkyl moieties such that each n-alkyl moiety has at least three carbon atoms. The present invention is also directed to bis-3,9-(2,6-di-t-alkyl-4-alkylphenoxy)-2,4,8,10-tetroxa-3,9-diphosphasprio[5.5]undecanes and polymer composition containing an effective amount of same.