Very few studies have investigated second language (L2) knowledge of word parts. The present study was set to measure L2 recognition of 35 English prefixes and the factors that might predict scores. The form and meaning sections of Sasao and Webb's Word Part Levels Test were administered to 152 Arabic learners of English as a foreign language (EFL), followed by a test of vocabulary breadth, and a short language background questionnaire. The factors we explored in the mixed-effects model analysis were the following: vocabulary breadth, duration of L2 exposure, and type/amount of contact with English in the four modalities. We additionally, explored whether Sasao and Webb's difficulty estimates reflect Arabic EFL learners' knowledge of prefixes. Results showed that vocabulary breadth significantly predicted both form and meaning knowledge of English prefixes. Moreover, weekly hours of reading and communication in English contributed significantly to prefix knowledge. Finally, Webb and Sasao's difficulty estimates predicted scores on both the form and meaning tests and interacted with vocabulary breadth, showing that the most difficult prefixes were challenging regardless of vocabulary breadth. These results are discussed in light of previous research on L2 morphological awareness, highlighting implications for teaching practices.