This paper presents context specific (classroom and test) psychometric data on Shell, Murphy and Bruning’s (1989) Writing Skills Self-efficacy Scale in a sample of 302 Australian middle school students. Students reported significantly lower writing self-efficacy for the test context than for the classroom context, supporting the need for context differentiation in the measurement of self-efficacy. Psychometric data on a new scale, the Sources of Writing Self-efficacy Scale, suggested a four-factor structure in line with Bandura’s (1997) theoretical sources of self-efficacy, that was confirmed in a follow up study among a sample of 307 Australian middle school students. Regression analyses indicated the predictive ability of the scale factors when regressed onto writing self-efficacy in both the classroom and test contexts. Implications for educators for molding the development of writing self-efficacy are discussed.