The 500V junction-isolated BiMOS high voltage IC process technology recently introduced by the General Electric Company(1,2) and now in manufacturing production provides the circuit designer with a myriad of low and high voltage devices. However, to exploit the full capability of this process and apply it efficienty, it is crucial to understand and model the devices it makes available. This paper describes specifically the high voltage NPN structure, its resulting equivalent model, and shows actual device data. HSPICE device models that accurately predict the behavior of low voltage components (which may be referenced at high voltages) as a function of temperature have been developed and will be presented. We will conclude by showing generic application examples that convey the flexibility and power of this process.