Input and interaction have received great attention in second language research due to their dominant role in second language teaching and learning. However, empirical studies that have examined their effects on the acquisition of Chinese as a second language remain scarce. This study fills this gap by comparing the effects of teacher-student interaction and student-student interaction on vocabulary acquisition in Chinese as a second language. Thirty-six adult beginners of Chinese completed ten weeks' study in Australia, in addition to pre- and post- tests and a background questionnaire. Selected participants attended a focus group interview. Statistical analyses show that both types of interaction facilitate learning but their effects depend on the mode of tests. There was no statistically significant difference between the teacher-student interaction and student-student interaction in their effects on the acquisition of word meaning based on written scores, whereas the teacher-student group outperformed the student-student group in pronunciation. There was no statistically significant difference between the written and spoken scores of the teacher-student group, whereas the student-student group had higher written scores than spoken scores. The results suggest when learners are allowed to select learning content and provided with the learning materials prepared by teachers, the student-student interaction may achieve the same results as teacherstudent interaction in the acquisition of word meaning. The findings lend support to the Input and Interaction Hypothesis by showing that negotiation of meaning and comprehended input facilitated vocabulary acquisition, and extend the effects of interactions to the acquisition of Chinese as second language by pure beginners using their first language. The findings and their pedagogical implications are discussed and contextualised within Chinese-as-a-second-language teaching.