Free and bound long-chain alkenones (C37:2 and C37:3) 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 U37k'. The profiles of both alkenones across the redox boundary show a preferential degradation of the C37:3 compared to the C37:2 compound. Because of the high initial U37k' values and the way of calculating the U37k' this degradation hardly influences the U37k' profiles. However, for lower U37k' values, measured selective degradation would increase U37k' up to 0.17 units, equivalent to 5 ¿C. For most of the U37k' band-width, much smaller degradation already increases U37k' beyond the analytical error (0.017 units). Consequently, for interpreting the U37k' record in terms of past sea surface temperatures, selective degradation needs serious consideration. ¿ 1998 American Geophysical Union