Abstract: A microprocessor based system for detecting leaks from underground gasoline storage tanks and the product lines through which the gasoline is pumped to above ground dispensers. Hydrocarbon sensitive probes spaced around the tank in monitoring wells detect gasoline leakage from the tank. Each probe can be independently programmed as to the threshold hydrocarbon concentration considered to be indicative of a leak. Pressure transducers sense the product line pressure and distinguish thermally induced contraction from true leaks by comparing the time required for a preselected pressure drop to occur during successive cycles. A time comparison is also used to detect the presence of excessive air in the product line. An alarm condition sensed by either type of sensor generates audible and visible alarm signals and automatically dials a programmed telephone number. The alarm times and dates are stored in memory and may be called up on the display screen.