In recent years ultra-high-performance concrete (UHPC) has gained more interest in the concrete construction industry due to the expected high durability of UHPC, as well as extended architectural opportunities. Compared to normal concrete it is possible to build filigree and lighter structures with ultra-high-performance concrete. The aim of this study was to evaluate the durability of different UHPC mixtures regarding alkali-silica reaction (ASR) and delayed-ettringite formation (DEF). UHPC prisms were exposed to different temperature and moisture conditions in a special climate simulation chamber. Scanning electron microscopy (SEM) was used to determine possible deterioration of UHPCs. Results of microscopic investigations show that products of ASR are only locally enriched. An ettringite growth was observed on microcracks (< 10 µm) in intentional pre-damaged samples or close to incompletely hydrated clinker grains. Macroscopic deteriorations due to ASR or ettringite growth could not be detected. However, steel fibers of UHPC were affected by corrosion.