We present 4 experiments investigating dynamic and flexible aspects of semantic activation spread during speech planning. In a semantic blocking paradigm, pictures of objects were presented in categorically homogeneous blocks consisting of semantic category members (e.g., foods), in blocks consisting of seemingly unrelated objects that could potentially be integrated into a common theme (e.g., fishing trip), or in heterogeneous blocks consisting of entirely unrelated objects. In Experiment 1 we observed a classic semantic interference effect for the categorically homogeneous condition but no effect for the thematically homogeneous condition. In Experiment 2 the blocks were preceded once by visually presented title words. When titles were presented that referred to the semantic category or theme of the block, interference was observed not only in the categorically homogeneous condition but also in the thematically homogeneous condition. The ad hoc semantic interference effects for thematic relations were replicated with a different set of materials in Experiments 3 and 4. These observations reveal the dynamic nature of the speech production system, shaped by context and formations of flexible ad hoc categories and semantic relations. (Contains 3 footnotes, 4 tables and 3 figures.)