Some strands of research on teacher cognition have remained predominantly cognitive and overlooked the socially-mediated nature of teachers' thoughts. One such strand is the research on teachers' pedagogical knowledge base, defined as the thought units/categories underlying teachers' instructional performance. To address this gap, the present study investigated L2 teachers' pedagogical knowledge base with a specific focus on online reading instruction at a tertiary education level and the antecedents of such pedagogical thoughts. To this end, three novice and three experienced teachers were recruited as participants. A complete online instructional session per teacher was video-recorded and used as a stimulus for inspecting their pedagogical thought units (PTUs) and the antecedents driving/shaping the thoughts. The findings revealed striking differences in the pedagogical thought categories (PTCs) of the two groups. The results further demonstrated that experienced teachers' pedagogical knowledge was informed by both cognitive and social sources, whereas novice teachers' thoughts were more cognitively oriented. In particular, experienced teachers highlighted prior teaching experiences, students' feedback, and colleagues' advice as the mediators guiding their PTUs. In contrast, novice teachers relied on knowledge from research-based theories and teacher education courses to inform their pedagogical thoughts.