The invention provides a heat transfer composition comprising (i) a first component selected from trans 3 3 3 tetrafluoropropene (R 1234ze(E)) 1 3 3 3 tetrafluoropropene (R 1234ze(Z)) and mixtures thereof; (ii) carbon dioxide (R 744); and (iii) a third component selected from difluoromethane (R 32) 1 1 1 2 tetrafluoroethane (R 134a) and mixtures thereof.