Skilled interpersonal communication improves individual and group outcomes in life and work. This makes it important to find effective and efficient training methods to teach interpersonal communication skills. Internet based interventions may provide training to a wide audience at low cost and have been shown to be effective for a range of health related behaviours, but so far not for communication skills training. The present study evaluates the efficacy of an internet-based intervention to teach interpersonal communication skills relative to a control group. Participants were randomly assigned to one of the two conditions. In the treatment condition, participants got access to a four week internet based communication skills training program. The control group was put on a waiting list. Interpersonal communication skills were measured at baseline and at 1 month, 2 months and 6 months follow ups, using a selection of subscales from the Conflict Resolution Questionnaire and the Perspective Taking scale. Analyses did not find any statistically significant effects of the intervention relative to the control group. Several possible reasons for this null finding are discussed, and directions for future research are proposed.