A mechanism is proposed to explain the experimental observations in the iodine-catalyzed R/S isomerization of the allenic carotenoids peridinin, fucoxanthin, and neoxanthin. Attack of I on C7', suggested to be the key step, breaks (or forms a partial bond with I in a transition state) the C7'-C8' or C7'-C6' pi bond, which allows subsequent R/S isomerization. AM1 calculations show that the charge density at C7' of both the 6'R-isomer and 6'S-isomer is the greatest among the polyene and allenic carbons, and that the electron densities in the p(x) and/or p(y) orbitals of C7' are also higher than for most other polyene and allenic carbons. Thus, h can preferentially attack C7', and significant R/S isomerization occurs. The calculation results can also explain the experimental observation that the ease of R to S isomerization of allenic carotenoids is peridinin > fucoxanthin > neoxanthin. Electrochemical experiments show that the radical cation and dication of fucoxanthin are unstable, which suggests that R/S isomerization of allenic carotenoids is not via their radical cations or dications. Furthermore, AM1 calculations show that R/S isomerization via the radical cations or dications is energetically unfavorable.