Among other technologies, wikis, as a Web 2.0 technology, have been found to support online collaborative behaviour of students in group work. Despite the intention-behaviour relationship expected in many relevant theories, studies have found that the relationship between students' intention to use wikis and their behaviour in using wikis was not strong. This discrepancy between expectation and actuality is referred to as the intention-behaviour gap. Researchers have explored mediators that can bridge the intention-behaviour gap. Given the study of behaviour across various disciplines, the variables that can bridge the intention-behaviour gap may be situational in nature. The present study therefore explored the effect of two mediators in a hypothesised model of the behaviour in using a wiki for students' group assignments. In a longitudinal study with a sample of university students in Hong Kong, factor-based partial least squares structural equation modelling was used to examine the measurement and structural models. The results indicate that goal commitment and wiki-based communication, while substantially increasing the combined explanatory power of the variance in wiki use behaviour, significantly mediated the path from intention to behaviour. Both practical and research implications have been provided in this paper.