Abstract: An apparatus for monitoring a lubricant in a machine comprises sensors S1, S2 for producing signals representing electrostatic activity in a machine lubricant and a temperature sensor for producing a signal representative of the temperature of the lubricant. A signal processor 9 is arranged to process the signals from the sensors S1, S2 to detect an electrostatic activity precursor indicative of an impending wear event in the machine. The signal processor is arranged to operate in response to the temperature sensor to compensate for temperature related changes to the signals from the electrostatic sensors S1, S2.