Abstract: Alarm-indicating gas detection apparatus including a first semiconductor gas-sensing element having one electrical property which varies significantly with increasing concentrations of a first undesirable gas and less significantly with increasing concentrations of a second undesirable gas and a second semiconductor gas-sensing element having the same electrical property which varies significantly with increasing concentrations of the second gas and less significantly with increasing concentrations of the first gas. The electrical property of both gas-sensing elements tends to vary similarly with age and with changing conditions such as temperature and relative humidity. A compensation system is used to cancel out errors due to variations in the electrical property. Threshold circuits are used to distinguish between first and second undesirable gases and to generate alarm signals.