The study explored the acquisition of L2 English non-null arguments by L1 Thai learners, that is, whether they were able to recognize and correct sentences with null arguments. Two variables which presumably affected the acquisition, namely clause types and prepositional phrases (PPs), were employed. A grammaticality judgment task (GJT) was administered to 31 intermediate and 31 advanced undergraduate L1 Thai students. The results suggested that sensitivity to recognize null arguments increased with higher proficiency. Paired-samples t-tests were performed to determine whether the two variables affected the acquisition. For the intermediate group, it was found that both clause types and PPs affected their judgments. We propose that perceptual salience could account for the intermediate participants' preference for null embedded subjects over null matrix subjects and that the lack of argument/adjunct knowledge could account for the intermediate participants' preference for null object sentences followed by a PP over null object sentences without a PP following. For the advanced group, it was found that clause types affected their judgment. We propose that informal styles of communication influenced the advanced participants' preference for null matrix subjects.