Phosphonio-substituted aldiminium, iminium and imidazolidinium salts are readily prepared in excellent yields by addition of phosphines to Alder’s dimer, or by treatment of the corresponding chloro iminium salt with the in situ generated phosphine/trimethylsilyl triflate adduct. A two-electron reduction, using either potassium metal or tetrakis(dimethylamino)ethylene, leads to the corresponding C-amino phosphorus ylides. When basic phosphine fragments are used, the ylides can be isolated; otherwise they spontaneously undergo a fragmentation into carbene and phosphine. Although the reduction/fragmentation sequence occurs under mild conditions, this method is limited to the preparation of transient carbenes, due to the non-availability of sterically hindered dications, and consequently of phosphorus ylides bearing bulky carbon substituents. To overcome this difficulty, a second novel route to phosphorus ylides has been developed: The addition, at low temperature, of 2,4-di-tert-butyl-ortho-quinone to readily available C-amino phosphaalkenes. Provided the phosphorus atom bears either an amino or tert-butyl group, a [4+1]-cycloaddition occurs, and the resulting ylides spontaneously undergo a fragmentation into a dioxaphospholane and a spectroscopically observed carbene.