The Collaborative International Dictionary
Diphthongally
Diphthongal \Diph*thon"gal\ (?; 115), a. Relating or belonging to a diphthong; having the nature of a diphthong. -- Diph*thon"gal*ly, adv.
Wiktionary
diphthongally
adv. By means of a diphthong.