Reliability aspects of Hybrid Microcircuits are considered, including a prediction model illustrating the detail knowledge of the technology and application necessary. Circuit, part and element data is presented for two different test circuits. Use is made of predicted circuit failure rates to establish an electrical endurance test programme together with the thermal stress levels likely to generate some failures in an acceptable time. Results of testing the circuits at more than one temperature are presented, evaluated and compared with prediction. Failure analysis enables the performance of add-on parts, circuit elements and bonds to be assessed. A second series of test circuits confirms an improved bonding system. The usefulness of reliability modelling prior to endurance testing is justified together with some degree of thermal overstress.