Noise reduction in compressors used in refrigeration systems is achieved by introducing between 0.01% and 10% by weight of a phosphate ester additive to the compressor lubricant. The phosphate ester additive is of formula (I) wherein each R is independently alkyl, aryl or ring-substituted aryl. Preferred phosphate ester additives are ring-substituted aryl esters. Noise level reductions of at least 1dB at a frequency between 0 and 20KHz are achieved.