Most of the models accounting for L2 oral production have deemed a significant role for vocabulary knowledge in this process. Previous studies have demonstrated a positive relationship between different aspects of lexical knowledge and performance or proficiency of second language skills including the speaking performance. Meanwhile, the findings have suggested a determining role for the task type used for measuring speaking performance when one or more aspects of lexical knowledge are in focus. This study was conducted to investigate the relationship between the EFL Learners’ deep vocabulary knowledge (DVK) and speaking performance by scrutinizing the mediating role of task type. To this end, 102 bachelor ELT students were given Word Associate Test to measure their DVK, and a planned presentation task and unplanned tasks of description, narration and reasoning to elicit speaking performance. The elicited samples of speaking performance were transcribed and analyzed in terms of fluency, accuracy, lexical complexity and grammatical complexity. Structural equation modeling indicated a lack of causal relationship between DVK and aspects of speaking performance as measured with both planned and unplanned tasks. However, mixed results were obtained in the case of the correlations of fluency, accuracy, grammatical complexity and lexical complexity with DVK across different tasks. Although the findings do not provide evidence for a strong relationship between DVK and speaking performance when DVK is analyzed in isolation from other aspects of vocabulary knowledge, the variation witnessed in findings provide further proof for the importance of task effectiveness in the study of lexical access.
