An acoustic temperature measurement system for pipeline fluids comprises a loudspeaker (16) with spaced microphone (18) in a sealed pipe (10) with end cap (11). A computer arrangement (22) provides a tone burst signal which drives the speaker via power amplifier (21) and the outgoing and reflected signal is detected by the microphone (18) and following amplification in amplifier (20) passes to the computer (22) for processing. The signals are processed to determine phase relationships indicative of temperature and can be used in correcting leakage measurement readings.