Dataset and Codebook for the study titled „Persistence of situational language balance in bilingual switching: Evidence from carryover of L1 slowing“ (Koch, Sánchez, Koch, Roembke, Philipp, & Declerck). Previous studies have demonstrated a so-called L1 slowing effect in language switching: although performance in naming tasks is generally better in the dominant first language (L1) than in the second (L2), this is often reversed in situations that require frequent language switching. This observation suggests that language control which proactively inhibits L1 during L2 naming is carried over into L1 naming. This study examined the persistence of the L1 slowing effect. Participants were 50 native speakers of German (18-54 years, 30 female/20 male), mostly Psychology students at RWTH Aachen University. They named non-cognate pictures in German (L1) and in English (L2) in single-language and in mixed blocks. A pretest consisting of two single-language blocks of 40 trials each was followed by four mixed-language blocks of 80 trials each. These were followed by a posttest which again consisted of two single-language blocks of 40 trials each. Block language order (English/German) within pre- and posttest was counterbalanced across participants.