Collaborative writing (CW) research has gained prevalence in recent years. However, the ways in which students interact socially to produce written texts through synchronous collaborative writing (SCW) is rarely studied. This study aims to investigate the effects of SCW on students' writing products and how collaborative dialogues facilitate SCW. Following an initial analysis, 54 students were divided into 18 groups; six groups with higher proportions of collaborative dialogue (HCD), six groups with median proportions of collaborative dialogue (MCD), and six groups with lower proportions of collaborative dialogue (LCD). The data collected includes the students' three reaction essays, their transcripts of text-based collaborative dialogues, and their writing process logs. The results showed that there were significant differences between the LCD, MCD, and HCD groups in terms of fluency and accuracy of their reaction essays. Through collaborative dialogues, students benefitted from text-based synchronous communications, such as clarifying their linguistic misconceptions, and receiving immediate feedback to help resolve their writing problems. The findings suggest that students could be provided with more opportunities for collaborative dialogues during the entire writing process, including the stages of generating ideas, writing reaction essays, and editing.