Communicative adequacy is a key construct in second language research, as the primary goal of most language learners is to communicate successfully in real-world situations. Nevertheless, little is known about what linguistic features contribute to communicatively adequate speech. This study fills this gap by investigating the extent to which complexity, accuracy, and fluency (CAF) predict adequacy, and whether proficiency and task type moderate these relationships. In all, 20 native speakers and 80 second language users from four proficiency levels performed five tasks. Speech samples were rated for adequacy and coded for a range of CAF indices. Filled pause frequency, a feature of breakdown fluency, emerged as the strongest predictor of adequacy. Predictors with significant but smaller effects included indices of all three CAF dimensions: linguistic complexity (lexical diversity, overall syntactic complexity, syntactic complexity by subordination, and frequency of conjoined clauses), accuracy (general accuracy and accuracy of connectors), and fluency (silent pause frequency and speed fluency). For advanced speakers, incidence of false starts also emerged as predicting communicatively adequate speech. Task type did not influence the link between linguistic features and adequacy.