Aside from learning the basic and common grammatical structures, English as foreign language learners ("hereafter EFL") have to be familiar with a wide range of lexical chunks called collocations. Being sensitive to collocations is the key of attaining a native-like fluency in English. The aim of this study is twofold: to explore the perceptions of collocations of a group of EFL students (N=19) who had studied English prep-class at a state university for a year and to investigate their collocational errors and the sources of them. The data were collected through an open-ended questionnaire, two focus-group interviews and EFL students' writings. Thematic analysis was utilized to analyze the data gathered from the questionnaire and focus-group interviews and students' writings were examined based on the taxonomy of collocation suggested by Benson, Benson and Ilson (2010). Although all participants were cognizant of the concept of collocation and its role in language learning, they made collocation errors when producing their writings due to the lack of automation of collocation mostly caused by L1 interference. We suggest that data-driven learning may be beneficial in helping EFL students to develop automaticity in the use of collocations. Additionally, corpus-based activities may provide effective authentic materials for them.