In short, our students do not know where to look when they cannot understand a piece of written text. An important issue, then, is the cohesive ties which have been found to be problematic for EFL readers (Zamel, 1983). Among cohesive devices references have a major contribution to textual cohesion in English, and as Cook (1989) states, many comprehension difficulties arise from the difficulties students have in interpreting the meaning of referring expressions within sentences. In this case, Selinker and Trimble (1974 as cited in Carrel, Devine, and Eskey, 1988: 152-153) carried out a detailed study into the use of vocabulary in ESP and reported, “students' difficulties in ESP were not merely a result of technical vocabulary. In fact, non-technical words in technical writing would sometimes give students more difficulty than technical ones- e.g., adverbial phrases, conjunctions, or words used in anaphoric reference”.