What is the plural of diocese? Easy, you might think – it’s dioceses. Ah yes, but how do you pronounce it? Until 1999, and the tenth edition of the Concise Oxford Dictionary, no help was given by most ‘ordinary’ dictionaries. They said nothing at all about the plural, with the implication that it was regular: di-o-ce-ses (4 syllables). This is still the case for the English Pronouncing Dictionary, and the Oxford Dictionary of Pronunciation. However, starting in 1990 with his first edition, John Wells noted in the Longman Pronunciation Dictionary that an alternative was to make an analogy with analysis ~ analyses.
An older spelling for diocese was ‘diocess’. If we had kept this spelling, perhaps the plural, ‘diocesses’, would have stood more chance of surviving. We already have abscess ~ abscesses, and process ~ processes.