What is with words that have forms that end both in -zation and -sation, such as localization and localisation?
Many spell checkers recommend -zation.
The “z” is American, the “s” is British. As an American I always use “z” for these words, and apparently many UK spellcheckers do not mark it as wrong and the Oxford English dictionary even gives “organization” as the first spelling, saying (also organisation).
Yet my (US English) Firefox marks the “organisation” variant as misspelled and suggests “organization”.
My advice: unless you are writing under guidelines that suggest otherwise, use the “z” form for these words.