The article shines light upon the differences in the vocabulary demands of academic spoken discourse between three broad scientific disciplines: Life Sciences, Physical Sciences, and Social Sciences. By employing the Academic Word List (AWL) and British National Corpus/Corpus of Contemporary American English (BNC/COCA) wordlist, the present study analyzed data of the transcripts from 160 university lectures, 39 seminars, and 600 TED talks. Results from the analysis of the 2.5-million-token corpus demonstrated an order of lexical difficulty in which Life Sciences and Social Sciences were the most and least lexically demanding fields of study, correspondingly. Research findings also indicated a strong supportive relationship between the AWL and the BNC/COCA wordlist. Learners with limited vocabulary knowledge at 1,000 and 2,000 levels could significantly increase their lexical coverage for academic lectures, seminars, and presentations with the support of the AWL. For disciplines with low lexical demands like Social Sciences, the vocabulary knowledge of nearly 2,570 word families (BNC/COCA 2,000 + AWL) could help students understand 95.07% of the running words in universities lectures and seminars. Research findings offer implications for vocabulary teaching and learning.