This paper describes a model for insulator surface charging in high‐voltage ceramic vacuum diodes. The model involves electron emission from the insulator‐cathode‐vacuum junction followed by electron avalanches on the insulator surface which leave the wall positively charged. Experiments were performed to measure (i) the triple‐junction (cathode‐vacuum‐insulator interface) emission and its relation to the initiation of the avalanche, (ii) the dynamic current in the avalanche, and (iii) the saturated surface charge resulting from the avalanche. The experimental results were interpreted by computer simulation in terms of the model and were found to be in close agreement with the predictions.