Individual differences in motivational beliefs, such as writing efficacy and attitudes toward writing in different contexts (academic, recreational, print or digital), are hypothesised to account for variability in writing performance. This study tested this proposition with 185 middle school students (58% were male), examining if writing self-efficacy and attitudes toward writing made a statistically significant and unique contribution to predicting performance on a norm-referenced writing measure, after variance related to gender, free/reduced lunch status, student's first language, disability status, reading self-efficacy and attitudes toward reading were first controlled. Collectively, writing attitudes and self-efficacy accounted for statistically significant and unique variance in writing scores after the other variables were controlled, providing support for the theoretical proposition that writing motivational beliefs are important ingredients in middle school students' writing.