This article proposes that creoles are relatively transparent compared to their source languages. This means that they display more one-to-one relations between meaning and form. Transparency should be distinguished from the concepts of simplicity, ease of acquisition, and regularity. Definitions of these notions are given and it is shown how they have been mixed up in earlier literature. The transparency of creoles is explained as a result of language contact. When people speaking radically different languages communicate, they tend to use maximally intelligible forms, i.e. transparent forms. The repeated selection of transparent over opaque forms will lead to the formation of a relatively transparent language. Hence, creoles are predicted to be either as transparent as or more transparent than their source languages. An empirical study is performed to test this prediction. The transparency of four contact languages and their sub- and superstrates is measured by checking them on a list of non-transparent features. It turns out that they all exhibit opaque structures, but that there is a striking absence of so called form-based forms: linguistic elements and rules that are not motivated pragmatically or semantically. This indicates that such ‘empty’ forms are lost during intense language contact.