Sensors commonly used in workplace multi-gas monitors, and normally operating at atmospheric pressure, have been evaluated under the pressurized (hyperbaric) atmospheres typically found in tunnelling operations, i.e., 1–4.5 bar (100–450 kPa) static pressure and pulses of up to 0.3 bar (30 kPa) above ambient pressure and of less than 30 s duration. Three types of sensor have been tested: catalytic sensors (pellistors) for methane, electrochemical fuel cells for carbon monoxide and electrochemical metal-air batteries for oxygen. All the sensors are affected by pressure to some degree. Pellistors show a positive pressure effect (response increases by up to 50% at 3 bar), while pulses up to 0.3 bar produce transient responses equivalent to 0.2% methane. The electrochemical carbon monoxide sensors are unaffected by pressure in clean air but there is a positive effect in carbon monoxide/air (the sensor reads approximately 11% high at 3 bar). During compression and pulsing in carbon monoxide/air larger effects are observed. The electrochemical oxygen sensor behaves similarly to the carbon monoxide sensor: at 3 bar the sensor reads 12% high, while pulses of the order of 0.1 bar above ambient induce a 25% increase in response. The effects of static hyperbaric pressure can be overcome by recalibration of the sensors at the working pressure. Transient effects on electrochemical sensors can be reduced by careful siting of the sensor away from pressure-pulse sources or possibly by the incorporation of auxiliary diffusion barriers into the sensor. The effects of pressure on the sensor characteristics are compared with theoretical models.