Although research on proficiency pairing for collaborative writing in second language (L2) or foreign language (FL) higher education has proliferated in the last few decades, scant attention has been paid to students' perceptions about how they are paired up. This case study revisited proficiency pairing in light of equity theory, examining two high proficiency Chinese EFL learners' perceptions of proficiency pairing and how their perceptions affected their actions in the collaborative writing process. The findings reveal that students with similar advanced L2 proficiency behaved differently during a collaborative writing process due to their different perceptions of proficiency pairing. This study also found that motivation, perceived self-value, and collaborative writing assessment were three major factors that shaped students' perceptions of proficiency pairing. This study provides useful insights into methods for pairing learners in L2 writing classes and improving teacher education in L2 higher educational contexts.