Ise or Ize
From Mechanics in Guide part of ABC of Plain Words by Sir E Gowers (1951)

On the question whether verbs like organise and nouns like organisation should be spelt with an s or a z the authorities differ. The Oxford English Dictionary favours universal ize, arguing that the suffix is always in its origin either Greek or Latin and in both languages it is spelt with a z. Other authorities, including most English printers, recommend universal ise. Fowler stands between these two opinions. He points out that the Oxford English Dictionary's advice over-simplifies the problem, since there are some verbs (e.g. advertise, comprise, despise, exercise and surmise) which are never spelt ize in this country. On the other hand, he says

"the difficulty of remembering which these ise verbs are is the only reason for making ise universal, and the sacrifice of significance to ease does not seem justified".

This austere conclusion will not commend itself to everyone. It does not do so to the authors of the A.B.C. of English Usage, who say roundly "the advice given here is to end them all in ise", a verdict with which I respectfully agree.