New words constantly occur in the English language since productivity is one of the four design features of human language. However, the new words are rarely entirely new coinages. Instead they are often formed by derivation, which means adding prefixes or suffixes to an already existing morpheme. My aim in this paper was to study words derived by the four negational prefixes de-, dis-, non- and un-, and I focused on the words where the prefix changes the word class of the morpheme it is attached to. Examples are degreen, where the prefix makes the adjective green into a noun, and non-stop where the verb stop is changed into an adjective. I wanted to find out if the prefixes follow ceratin patterns and if some of the four prefixes are used more creatively and more often than the others. In order to get a good sample of the english language today, I used a corpus consisting of texts from the Guardian/the Observer. And to decide whether to regard a word as new or not I used various dictionaries including some published on the web. The results show that de- and non- are used more creatively than dis- and un-, and they also occur in more words where a change of word class has taken place than the others. The most common type of word class change was to form verbs from nouns, for example debug and defrost, and de- was the prefix that dominated this group.