A process for the manufacture of retinal (I) comprises reacting a 5-(2,6,6-trimethyl-cyclohex-1-enyl)-1,4-pentadiene derivative (IIa) or a 5-(2,6,6-trimethyl-cyclohex-2-enyl)-1,4-pentadiene derivative (IIb) or a 5-(2,6,6-trimethyl-2-cyclohexen-1-ylidene)-1-pentene derivative (IIc) or a 5-(2,6,6-trimethyl-cyclohex-1-enyl)-penta-1-en-4-yne derivative (IId) or a 5-(2,6,6-trimethyl-cyclohex-2-enyl)-penta-1-en-4-yne derivative (IIe) with a 1,3-butadiene derivative H2C.dbd.C(CH3)CCH.dbd.CHOR4 (III) in the presence of a Lewis or Bronsted acid and subjecting the compound obtained in each case [(IVa), (IVb), (IVc), (IVd) or (IVe), respectively] to basic or acidic conditions to eliminate therefrom the moiety R2H and thus produce, according to the immediate precursor, retinal itself or a particular derivative thereof [(I'), (I''), (Va) or (Vb), respectively] and, where in two cases such derivative is produced featuring a triple bond [derivative (Va) or (Vb)], hydrogenating this to produce retinal (I) or the derivative (I'), respectively, and each case where a derivative (I') or (I'') has been produced, isomerizing this under basic or acidic conditions or in the presence of a metal catalyst to the desired retinal (I). The so-produced retinal is usually in the form of an isomeric mixture, normally as (9 E/Z, 13 E/Z)-retinal, and this can be isomerized according to a further inventive aspect to (all-E)-retinal by the acid-catalyzed formation of an adduct of (all-E)-retinal with hydroquinone in crystalline form. The so obtained (all-E)-retinal-hydroquinone adduct can then if desired be converted to vitamin A alcohol in the predominantly (all-E)-isomeric form by a method known per se. The novel starting materials (IIa), (IIb), (IIc), (IVI) and (IIe) represent a still further inventive aspect. Retinal is a valuable intermediate in the synthesis of further vitamin A compounds (retinoids). The retinoids, particularly vitamin A alcohol (retinol), are known to be valuable substances which promote the well-being of humans, inter alia in respect of vision, the immune system and growth, and for this reason are often used as components of multivitamin preparations and as additives for certain food- and feedstuffs.