The ability of glyoxysomes from sun¯ower (Helianthus annuus L.) cotyledons to completely degrade long-chain fatty acids into their constituent acetyl units and the time courses of the appearance of acyl-CoA intermediates during b-oxidation have been studied using 14 C-labelled substrates at non-saturating concentrations (1.3 to 1.8 lmol á l A1 ). [ 14 C]Acetyl-CoA was formed from [18-14 C]oleate metabolized at a yield of up to 80%, and from [U-14 C]palmitate and [U-14 C]linoleate to an extent indicating that a maximum of 80% and 30%, respectively, of the substrate b-oxidized had been degraded beyond the C 4 -CoA intermediate level. To obtain the latter values, an acetyl-CoA-removing system was required during b-oxidation. Constant re-oxidation of the NADH formed during the b-oxidation did not replace the eect of acetyl-CoA removal. Neither the completeness of the linoleate b-oxidation nor the rate of reaction were in¯uenced by NADPH. Medium-and short-chain acyl-CoA intermediates were predominantly detected during b-oxidation of the long-chain substrates employed. The degradation of these intermediates appeared to be stimulated mainly in the presence of an acetyl-CoA-removing system. The time courses of the appearance of intermediates corresponded to a precursor-product relationship between intermediates of decreasing chain lengths.