Chemistry occurring on asteroids, comets, in the interstellar medium or on other planets.
Current astrochemical research has been directed at interstellar ice and dust grains; crystals of volatiles that can take hundreds of millions to form. This gives rise to myriad crystal types not often seen on Earth owing to the very specific conditions required for their formation (extremely low temperatures being one such criterion).
Further topics might include heterogeneous catalysis on these icy grain surfaces, a cause of some very unique interstellar chemistry (see Laboratory investigations of the role of the grain surface in astrochemical models for more info). Life on other planets is also another subject, although still of a highly speculative nature.