source: oxforddictionaries.com
Many verbs that end in -ize in American English can also end in -ise in British English. For example, in British English, all of the following spellings are acceptable: finalize/finalise; organize/organise; realize/realise. In American English, the only correct spellings are finalize, organize, and realize.
In both American and British English, there are a small set of verbs that must always be spelled with -ise at the end. The main reason for this is that, in these words, -ise is part of a longer word element rather than being a separate ending in its own right. For example: -cise (meaning 'cutting) in the word excise; -prise (meaning 'taking') as in surprise; or -mise (meaning 'sending') in promise.
Here are the most common verbs that must end with -ise:
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advertise
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compromise
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exercise
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revise
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advise
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despise
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improvise
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supervise
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apprise
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devise
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incise
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surmise
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chastise
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disguise
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prise (meaning ‘open’)
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surprise
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comprise
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excise
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promise
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televise
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There are also a few verbs that always end in -yze in American English:
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analyze
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catalyze
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electrolyze
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paralyze
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breathalyze
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dialyze
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hydrolyze
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psychoanalyze
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In British English, they are all spelled with the ending -yse (i.e., analyse, breathalyse, catalyse, etc.).
