Whereas triphenylphosphonioethanide manifests unsatisfactory cis-selectivities (84.0-95.5%), tris(o,o'-difluorophenyl)- and tri-o-tolylphosphonioethanide react with aliph. or arom. aldehydes and, resp., 2-alkenals or 2-alkynals to afford the olefins with cis/trans-ratios of 99:1 and 96:4 (or better). Triphenylphosphoniobutanide like other higher-homolog std. ylides shows excellent stereoselectivity (99% cis) when condensed with aliph. aldehydes but fails to produce cis-olefins derived from arom. aldehydes with >96% or 97% isomeric purity. In such cases tris(o,o'-difluorophenyl)phosphoniobutanide gave a 99% level of selectivity. [on SciFinder (R)]