High-pressure ethene polymerizations using thirteen tert-alkyl peroxyester initiators, R-C(CH3)2-O-O-CO-R' (with R = methyl, ethyl, neo-pentyl, tert-butyl and R' = methyl, i-propyl, propyl, butyl, tert-butyl, 2-heptyl) have been carried out in a CSTR. The initiator efficiency f has been determined for each peroxyester at 2 000 bar over a range of temperatures, partly up to 250 °C. The highest initiator efficiencies, up to about f = 0.9, are observed for tert-butyl peroxyesters, in which the R' species is a primary radical, as e.g., with tert-butyl peroxypentanoate and tert-butyl peroxyacetate. The lowest efficiency value, of about f = 0.37, occurs with tert-butyl peroxypivalate. The radicals from initiator decomposition, R'COO· and R-C(CH3)2O., may undergo β-scission reactions to produce R' radicals by decarboxylation and R radicals by “deacetonization”, respectively. In-cage termination reactions between oxygen-centered and carbon-centered radicals essentially control initiator efficiency.