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You often hear people say that -ize is American and -ise is British. While it is certainly true that -ise has become dominant in Britain, -ize is the older spelling, and it once was dominant everywhere. Americans have retained it, while the British have increasingly – since the mid-20th-century I think – favoured -ise. Why, I don't know.

Last time I checked, both the (British) Oxford and Collins dictionaries gave -ize as the first spelling.

I'm British and I always use the -ize spelling, not least because the letter z is so underused, but also because it more accurately indicates its pronunciation (and -ise is plain ugly).

In publishing, we usually call these -ize and -ise spellings. Some UK publishers prefer -ize, but -ise is the predominant UK style.

See Oxford spelling on Wikipedia. OED prefers -ize spellings.

The US requires -ize. The UK accepts both, but (with the OUP as the main exception) prefers -ise. And this is one of the few cases where "international" English follows the US convention. The ISO favours -ize spellings.

Inbuilt computer dictionaries set to UK English rarely support -ize. Feel free to overrule them.

That is because many spell checkers use American English.

Basically, z is used in North American English (American and Canadian), while s is used in British spellings. You can find explanations for this on the internet — like here, for example: http://www.studyenglishtoday.net/british-american-spelling.html

Which spelling you use, depends on who your audience is. If your writing is to be read mainly or only by speakers of American or Canadian English, use the North American spelling. If it is not, then use the British spelling.

Apparently, if you see a process of transformation, or as Dictionary.com defines the suffix, "1. To cause to become, resemble or agree with" the word should end with -ize, originating from the Late Latin -izāre, and the greek izein. Or -ize is the rule if there is a smaller word prior to the suffix. -ise is apparently from the latin past participle of words for "take", "see", and "cut" Analyse, for example, comes from (lysis) meaning dissolution. See http://theconversation.com/its-time-to-recognize-and-internalize-the-us-suffix-ize-19828

The New Oxford American Dictionary reports both the words, and the spelling checker used from the Mac OS X (10.6) corrects me when I write organisation.

Considering that US English is set as first language (followed by British English), I would think that organization is the American English word (even though it is not reported by the New Oxford American Dictionary).