This paper examines the use of English determiners in chat with the focus on determiner-related errors. For this, the comparisons were made in the use of determiners between chat and the grammatical judgment test, which was followed by the investigation of the role of task type on the determiner use in chat. The results show that deletions of determiners are significantly more common in the meaning-focused activity (i.e. chat), and that students commit more errors in the form-focused activity (i.e. the grammatical judgment test). It was also found that task type was a significant factor to cause the difference in the frequency of determiners, implying that the convergent task might be more appropriate to use to look at students’ competence regarding determiner use than the divergent task. However, the overall ratio of deletions was so high that it seems to be hard for researchers to obtain the precise picture of learners’ developmental state through the chat transcript.