Smoke has long been recognized as the most common source of 5re damage to electrical equipment; however, most failures have been analysed after the 5re was out and the smoke vented. The e4ects caused while the smoke is still in the air have not been explored. Such e4ects have implications for new digital equipment being installed in nuclear reactors. The US Nuclear Regulatory Commission is sponsoring work to determine the impact of smoke on digital instrumentation and control. As part of this programme, Sandia National Laboratories has tested simple active circuits to determine how smoke a4ects them. These tests included the study of three possible failure modes on a functional board: (1) circuit bridging, (2) corrosion (metal loss), and (3) induction of stray capacitance. The performance of nine di4erent circuits was measured continuously on bare and conformally coated boards during smoke exposures lasting 1 h each and continued for 24 h after the exposure started. The circuit that was most a4ected by smoke (100% change in measured values) was the one most sensitive to circuit bridging. Its high impedance (50 M ) was shorted during the exposure, but in some cases recovered after the smoke was vented. The other two failure modes, corrosion and induced stray capacitance, caused little change in the function of the circuits. The smoke permanently increased resistance, of the circuit tested for corrosion, implying that the contacts were corroded. However, the change was very small ((2%). The stray-capacitance test circuit showed very little change after a smoke exposure in either the short or long term. The results of the tests suggest that conformal coatings and type of circuit are major considerations when designing digital circuitry to be used in critical control systems.