The ylides Ph(3)P=(CArAr2)-Ar-1 1a-g (Ar C6H5, p-C6H4Cl p-C6H4F, m-C6H4CF3, P-C(6)H4OCH(3), p-C6H4CH3) were treated with elemental selenium (similar to 80 degrees C) to give the corresponding selenoketones Se=CAr(1)Ar2 2 by Staudinger-chalcogenation. Their reaction with trans,trans-2,4-hexadiene proceeds completely stereospecifically to yield the 2,2-diaryl-3,6-dihydro-cis-3,6-dimethyl-2H-selenapyrans 3. In contrast, the reactions of the selenoketones 2 with cis, trans-2,4-hexadiene proceeds stereoselectively, also giving the dihydro-cis-dimethyl-2H-selenapyrans 3 as the major products, now admired with small amounts of the dihydro-trans-dimethyl-2H-selenapyran isomers 4. The [4 + 2] cydoaddition of 2 with cis,trans-2,4-hexadiene proceeds stereospecifically, however, when carried out at a pressure of 12 kbar, now yielding 4 as the major products along with the corresponding tetraarylethenes 8. Along with the results of additional mechanistic studies (determination of solvent and substituent effects) it can be concluded that diarylselenoketones are likely to react by means of a concerted [4 + 2] cycloaddition with very reactive conjugated dienes (such as trans,trans-2,4-hexadiene), whereas a stepwise mechanism, resulting in diene cis/trans-isomerization with subsequent mechanistic "leakage" to the concerted pathway, appears to be preferred when a much less reactive conjugated diene such as cis,trans-2,4-hexadiene is employed. The reaction of the corresponding diarylthioketones 5a-g with trans,trans- and cis,trans-2,4-hexadiene, respectively, shows an analogous behavior.