The present invention relates to a process for preparing a carotenoid mono-ester of the formula (I), including the stereoisomers of formula (I), wherein, R1 is e.g. hydrogen, C1-C20-alkyl, C2-C20-alkenyl, C4-C20-alkdienyl, C6-C20-alktrienyl or C8-C20-alktetraenyl, R2 is e.g. hydrogen or -NRaRb, wherein Ra is e.g. hydrogen, C1-C4-alkyl, -C(O)-C1-C3-alkyl, -Boc or -Cbz, Rb is e.g. hydrogen or C1-C4-alkyl, R3 is e.g. hydrogen, X1, X2 are independently from each other CH2 or C=O, which comprises reacting a phosphonium salt of the formula (II) or a stereoisomer thereof: wherein X1, R1, R2, and R3 are as defined for formula (I), R4 is phenyl, tert-butyl or tolyl, and Y- is a suitable counteranion, with a 12'-apocarothenal of the formula (III) or a stereoisomer thereof: wherein X2 is as defined for formula (I), in the presence of a base or a cryptobase.