yse and yze endings

In UK style, you should never encounter an ‘yze’ ending; for example, ‘analyse’, ‘paralyse’ and ‘electrolyse’ are UK spelling, no matter whether ‘ise’ or ‘ize’ endings are being used. In US style, these words would attract a ‘z’, so ‘analyze’, ‘paralyze’ and ‘electrolyze’.

See Butcher’s Copy-editing, section 6.14, for more examples. It also gives a useful list of words, such as ‘comprise’, that should always be spelt with an ‘ise’ ending regardless of overall ending style.