Mammoth Uncensored Word List:
32 words ending with ms, 17 letters long

anthropocentrisms anthropomorphisms antievolutionisms antireductionisms authoritarianisms collaborationisms communitarianisms configurationisms controversialisms countercriticisms counterterrorisms electromagnetisms electrooculograms electropherograms environmentalisms epiphenomenalisms gynandromorphisms hyperpituitarisms institutionalisms internationalisms introspectionisms mesembryanthemums multiculturalisms necessitarianisms premillennialisms pseudoclassicisms psychopannychisms spectroheliograms superromanticisms supranationalisms transnationalisms ultranationalisms