Many language instructors focus on vocabulary word by word, neglecting common phrases. The result is that English as a Second Language students do not learn to speak idiomatic English (i.e. they make collocation errors). This study of the English compositions of National Tainan Second Senior High School students in Taiwan examined collocation errors, categorizing them according to Benson, Benson and Ilson's Collocation Classification System. An examination was then made of the error types as correlated with general English proficiency.