A refrigerator working fluid which comprises(a) tetrafluoroethane, and(b) a synthetic ester lubricant with a viscosity of about 150 cSt or less at 40.degree. C. and having the formula (R.sub.1 -R.sub.2).sub.n R.sub.3wherein R.sub.1 is ##STR1## in which R.sub.5, R.sub.6 and R.sub.7 are each independently hydrocarbyl group of from 1 to 7 carbon atoms with the proviso that the sum of carbon atoms in R.sub.5 +R.sub.6 +R.sub.7 is from 3 to 9,R.sub.2 is selected from the group consisting of (CH.sub.2 --CH.sub.2 --O--).sub.b, ##STR2## and mixtures thereof, where b is an integer from 1 to 3, R.sub.3 is a monobasic or polybasic carboxylic acid radical of the formula ##STR3## where R.sub.4 is a hydrocarbyl group having m carbon atoms where m is an integer from 1 to 9, andn is an integer from 1 to 4.