It has been suggested that the structural composition of mixed languages and the linguistic processes through which they emerge are to some extent predictable, and that they therefore constitute a language “type” (e.g. Bakker and Mous, 1994b; Bakker and Muysken, 1995). This view is challenged here. Instead, it is argued that the compartmentalisation of structures observed in mixed languages (i.e. the fact that certain structural categories are derived from one “parent” language, others from another) is the result of the cumulative effect of different contact mechanisms. These mechanisms are defined in terms of the cognitive and communicative motivations that lead speakers to model certain functions of language on an alternative linguistic system; each mechanism will typically affect particular functional categories. Four relevant processes are identified: lexical re-orientation, selective replication, convergence, and categorial fusion. Different combinations of processes will render different outcomes, hence the diversity of mixed languages as regards their structure, function, and development.