From the awesome Oxford Dictionaries Blog, taken as is, to have when asking myself this, again:

-Ize or -ise?

Many people visiting the World (non-US) version of our website ask us why we spell words such as realize, finalize, and organize with ‘-ize’ spellings, rather than ‘-ise’. There’s a widespread belief that these spellings belong only to American English, and that British English should use the ‘-ise’ forms instead, i.e. realise, finalise, and organise.

In fact, the ‘-ize’ forms have been in use in English spelling since the 15th century: they didn’t originate in American use, even though they are now standard in US English.  The first example for the verb organize in the Oxford English Dictionary is from around 1425, from an English translation of a treatise on surgery written by the French physician Guy de Chauliac:

The brayne after þe lengþ haþ 3 ventriclez, And euery uentricle haþ 3 parties & in euery partie is organized [L. organizatur] one vertue.

The OED’s earliest example for realize dates from 1611: it’s taken from a definition in A Dictionarie of the French and English Tongues, a bilingual dictionary written by Randle Cotgrave:

Realiser, to realize, to make of a reall condition, estate, or propertie; to make reall.

The first recorded use of the verb with an ‘-ise’ spelling  in the OED is not until 1755 – over a century later!

The use of ‘-ize’ spellings is part of the house style at Oxford University Press. It reflects the style adopted in the first edition of the Oxford English Dictionary (which was published in parts from 1884 to 1928) and in the first editions of Hart’s Rules (1904) and the Authors’ and Printers’ Dictionary (1905). These early works chose the ‘-ize’ spellings as their preferred forms for etymological  reasons: the -ize ending corresponds to the Greek verb endings -izo and –izein.

The situation is slightly complicated by the fact that certain verbs must always be spelled with ‘-ise’ at the end in British English, rather than ‘-ize’: this is generally because they have come into the English language in a different way. You can also check out a list of these verbs. The difficulty in remembering which words belong to this group is perhaps one of the reasons that –ise spellings were adopted more widely in British English.

The dictionary on the UK/World side of our website gives alternative ‘-ise’ spellings at the main entries for all ‘-ize’ words where it’s appropriate. In British English, it doesn’t matter which spelling convention is chosen: neither is right or wrong, and neither is ‘more right’ than the other. The important thing is that, whichever form you choose, you should use it consistently within a piece of writing.