A process for preparing carotenoids, in which the process includes reacting a dialkoxy dialdehyde in a double Wittig condensation with a phosphonium salt of or in a double Wittig-Horner condensation with a phosphonate. The carotenoids include, for example, .beta.-carotene, zeaxanthin, canthaxanthin, astaxathin, lycopene and croceptin, which are useful as nutraceuticals, food colorants, and feed additives.