Reading is recognized as being the most important skill needed by ESP learners in their field of study, and vocabulary knowledge is the most widely discussed component of effective ESP reading per se. However, research on how much the different types of words exert substantial influences over ESP reading comprehension remains scanty. To address this lacuna, the present study aimed to examine the degree of contribution made by general high-frequency, core-academic, and technical-academic words to 127 Iranian learners studying Psychology at three state universities in Tehran, Iran. Three researcher-made and validated tests were used to measure the three aforementioned types of vocabulary knowledge accompanied by an ESP reading test. Data analysis using multiple regression revealed that the core-academic words and technical-academic words significantly contributed to the ESP reading comprehension, explaining about 92% of the variance in reading scores, but knowledge of general high-frequency words was not a significant contributor. Moreover, teaching core-academic and technical-academic words did not have a significant effect on ESP reading comprehension in the short term, yet a low significant difference was observed for technical academic words in the long term. Findings of this study imply that direct teaching of the core-academic and technical-academic words can help ESP learners improve their reading over time. The article ends with pedagogical implications and suggestions for future research.
