Mammoth Uncensored Word List:
23 - 20 letter words starting with i

immunocytochemically immunohistochemistry immunoprecipitations inappreciativenesses incommensurabilities incomprehensibleness inconsequentialities inconsiderablenesses indestructiblenesses indiscriminatenesses indistinguishability institutionalisation institutionalization intellectualizations interchangeabilities interconnectednesses internationalisation internationalization interstratifications irreconcilablenesses irreproachablenesses isopropylcyclohexane isopropylcyclohexene