In a lubricant-cooled gas compressor (10) the temperature of separated lubricant returned to the air-side (11) is monitored by a thermostatically controlled restrictor valve (22) which minimises the flow of returned lubricant when temperature is low as on start up or when running on reduced load but increases the flow as the lubricant temperature increases. This will minimise he condensation of water from indrawn air when the temperature of the compressed air is too low to retain the water as vapour, so that the returned lubricant is contaminated with water which will damage the moving components of the air-end.