Free and “bound” long-chain alkenones (C_37∶2 and C_37∶3) in oxidized and unoxidized sections of four organic matter-rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of U₃₇^k′. The profiles of both alkenones across the redox boundary show a preferential degradation of the C_37∶3 compared to the C_37∶2 compound. Because of the high initial U₃₇^k′ values and the way of calculating the U₃₇^k′ this degradation hardly influences the U₃₇^k′ profiles. However, for lower U₃₇^k′ values, measured selective degradation would increase U₃₇^k′ up to 0.17 units, equivalent to 5°C. For most of the U₃₇^k′ band-width, much smaller degradation already increases U₃₇^k′ beyond the analytical error (0.017 units). Consequently, for interpreting the U₃₇^k′ record in terms of past sea surface temperatures, selective degradation needs serious consideration.