Abstract: This paper argues for the value of system networks as a diagnostic tool in L2 writing education. We report on an investigation of Chinese students learning to write in English over a period of a year in Year 9 of high school, characterizing their choices in the system of modality in different writing tasks over that period, and we also report on an extension of this investigation based on two additional sampling points, Year 12 of high school and Year 3 of university. Our findings indicate that even when we sample L2 texts from three different periods in education, the expansion in the use of the meaning potential embodied in the system of modality is quite limited – as long as we focus on texts from one register. We have also seen from Xuan’s (2015. A longitudinal study of Chinese high school students learning English based on systemic functional text analysis. The Hong Kong Polytechnic University PhD thesis) study of Year 9 writing tasks from a few different registers that there was no clear sense of progression throughout the year. The results show that careful selection and ordering of texts from registers that afford learners with complementary opportunities over time to engage with and master different parts of the resources of the system of modality can be recommended in L2 writing education.