Recent reports have suggested the use of the relative abundances of longchain (n-C37) alken-2-ones (1, 2, and 3) in Quaternary marine sediments as a palaeoclimate indicator1. Here we report the synthesis of these ketones with all-E and all-Z stereochemistry. Use of the synthesised compounds has shown that the natural counterparts in the alga and in marine sediments have the unusual and unexpected