Purpose: To evaluate the effects of pretreatment (silanization) and thermocycling on bond strengths of 2 core materials to 3 different types of fiber posts. Materials and Methods: Bond strengths of prefabricated glass (group 1), quartz (group 2), and individually formed glass fiber posts (group 3) to either a flowable or a highly viscous composite resin core material were measured using a pushout design. All posts were investigated with or without the application of a silane. Half of the specimens were stored in distilled water for 28 days, and were subsequently subjected to thermocycling (5000 cycles, 5 to 55°C), whereas the other half was investigated after 24-h water storage. Results: Bond strengths were significantly affected by thermocycling (p 0.0005), post type (p 0.0005), and pretreatment (p 0.0005), but not by the core material (p = 0.149; four-way ANOVA). Posts of group 3 demonstrated significantly higher bond strengths than all other posts, even after thermocycling (p 0.05; Tukey-B). Conclusion: While silanization had negligible effects, thermocycling drastically reduced bond strengths regardless of pretreatment, post type, or core material. The higher bond strengths between the group 3 posts and the flowable composite resin core material might result in improved stability of the core for postendodontic restorations. Keywords: fiber posts, composite resin core materials, push-out bond strengths, silanization, thermocycling