This study explores cross-age peers' tutoring behavior to support freshmen collaborating online. The study fits in with the need to inquire into the process of peer facilitation in CSCL-environments and focuses on types of peer support and on the evolution over time. The study was conducted with 19 pairs of fourth-year students, each tutoring one asynchronous discussion group of about 10 freshmen during one semester. A content analysis scheme was developed to analyze tutors' contributions [Weinberger & Fisher (2006). "Computers & Education," 46, 71-95; Garrison et al. (2000). "The Internet and Higher Education," 2, 87-105]. Full transcripts were coded and units of meaning were chosen as units of analysis. As to the types of peer support, the results reveal a variety of tutoring interventions. Most often tutors provide organizational and social support. Further, they facilitate knowledge construction and concentrate on elucidating learning contents. Only exceptionally tutors engage in off-task talk. As to the evolution over time, the results show a gradual decline in the amount of tutor contributions. Further, multinomial logistic regressions reveal variation in the occurrence of different types of support. Peer tutors did however not evolve from modeling high-quality discussion behavior to eliciting this in the tutees (coaching). At all times tutors preferred addressing their interventions to the entire group.