Thermal decomposition of the 3-phenyl-substituted 2H-azaphosphirene complex 1 in the presence of dimethyl cyanamide and dimethyl acetylenedicarboxylate (DMAD) yielded dimethylamino-substituted products, the 2H-1,2- azaphosphole complex 4b and the diastereoisomeric Δ3-1,3,2-oxazaphospholene complexes 5a,b; this represents the first example of 1,3-dipolar cycloaddition reactions of a nitrilium phosphane ylide complex that is generated in situ by transylidation.