New kinds of hermetic gas-tight ceramic microreactors for high temperatures, harsh environments, and high pressures are presented. Besides the special manufacturing procedure and design possibilities, application possibilities are also outlined. The results are complemented with examples of simulation of temperature distribution, measurement of K-values, and thermal efficiencies on a specific cross-flow reactor with internal manifolds. The high thermal conductivity of the ceramic results in a superior gas-to-gas heat transfer and in reasonable efficiencies.