Reactions of Ph₃GePCO with KP(t-Bu)₂ and Ph₃SiCl, or direct reaction with Ph₃SiP(t-Bu)₂ provides the (Z)- and (E)-isomers of the phosphaalkene (t-Bu₂)PC(OSiPh₃)P(GePh₃) 2, respectively. These isomers interconvert thermally and photochemically, while 2 also undergoes silyl and phosphide exchange with silylphosphines, consistent with a mechanism involving the reversible silylphosphination of Ph₃GePCO.