Forty bilingual Ss were tested in four experiments to see if decoding of linguistically mixed texts with one-word substitutions in the other language would take more time than decoding of unilingual material. The overall aim was to further explore the nature of the language switch mechanism as it functions in input. In the first experiment, consisting of two unilingual and three bilingual sets of sentences, presented visually, Ss had to judge the trueness or falsehood of each statement. In the second study, Ss read unilingual and bilingual passages where reading times were compared. In the third experiment, patterned after Experiment I, Ss heard and judged the trueness or falsehood of forty sentences with randomly placed lexical substitutions. In the final study, the one-word substitutions, previously randomized, were placed at subject, then verb, phrase boundaries. Conclusions were that: (1) in input, switching occurs when more than one word is substituted, and this switching is primarily at the phonological and syntactic levels; (2) the switch mechanism operates mainly in output where mixing of both languages is deliberate; and (3) the bilingual possesses not two but one basic internal dictionary where words in both languages are pooled. (Author/CLK)