AbstractPhosphonio‐substituted aldiminium, iminium, and imidazolidinium salts are readily prepared by the addition of phosphines to the Alder dimer or by treatment of the corresponding chloroiminium salt with the phosphine/trimethylsilyl triflate adduct generated in situ. Reduction with 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 fragment into the carbene and phosphine. This method is limited to the preparation of transient carbenes, owing to the unavailability of sterically hindered dications, and consequently of phosphorus ylides with bulky carbon substituents. This difficulty is overcome by the addition of 2,4‐di‐tert‐butyl‐ortho‐quinone to readily available C‐amino phosphaalkenes at low temperature. Provided the phosphorus atom bears either an amino or tert‐butyl group, [4+1] cycloaddition occurs, and the resulting ylides fragment into a dioxaphospholane and a spectroscopically observed carbene.