This study examines several linguistic factors as possible contributors to perceived word difficulty in second language learners in an experimental setting. The investigated factors include: (1) frequency of word usage in the first language, (2) word length, (3) number of syllables in a word, and (4) number of consonant clusters in a word. Word frequency is often treated as the quantifiable correlate of word familiarity, and word length and number of syllables measure structural complexity of a word. Consonant clusters were introduced as the measure of phonetic complexity. A total of 217 native speakers of Spanish and Portuguese were given a vocabulary identification task in which they had to determine whether the words were 1) "Easy to learn," 2) "Difficult to learn," or 3) "Unknown." The findings showed that there is a correlation between English word frequency and perceived word difficulty of the ESL learners. In contrast, there were no clear effects of the other factors on perceived difficulty when the words were controlled for frequency. [For full proceedings, see ED564162.]