An Electronic Support Measures (ESM) system consists principally of a passive radar receiver and a deinterleaver. It measures the monopulse parameters of intercepted radars, sorts them into individual radars, identifies those radars, and passes their identities to an Electronic Counter Measures (ECM) system for further action. When the pulse arrival rate is too high for an ESM system, it will skip some pulses and consequently the reliability of radar identification will be decreased. From queueing theory, this paper develops an expression that relates the reliability, quantified as a figure of merit called the factor of successful processing (F,), to the pulse arrival rate and the service times of the ESM system. An on-line method to measure F., is also given, which shows that the measurements are in close agreement with the theoretical values.