The current study investigated the possibility that language switches could be relatively automatically triggered by context. "Single-word switches," in which bilinguals switched languages on a single word in midsentence and then immediately switched back, were contrasted with more complete "whole-language switches," in which bilinguals completed a full phrase (or more) in the switched to language before switching back. Speech production was elicited by asking Spanish-English bilinguals to read aloud mixed-language paragraphs that manipulated switch type (single word, whole language), part of speech (switches on function or content words), and default language (dominant language English or nondominant Spanish). Switching difficulty was measured by production of translation-equivalent language intrusion errors (e.g., mistakenly saying "pero" instead of "but"). Controlling for word length (more errors on short vs. long words), intrusions were produced most often with function word targets in the single-word switch condition, and whole-language switches reduced production of intrusion errors for function but not content word targets. Speakers were also more likely to produce intrusions when intending to produce words in the dominant language--a reversed dominance effect. Finally, switches out of the default language elicited many errors, but switches back into the default language rarely elicited errors. The context-sensitivity of switching difficulty, particularly for function words, implies that some language switches are triggered automatically by control processes involving selection of a default language at a syntactic level. At a later processing stage, an independent form-level monitoring process prevents production of some planned intrusion errors before they are produced overtly.