The ring strain present in 2-aza-1-phosphabicyclo[n.1.0]alka(e)nes (n = 1-5) is calculated at the G3(MP2) level using homodesmotic reactions. The influence of cyclopropa(e)nation and heteroatom substitution is analyzed by a comparison with the corresponding bicyclic hydrocarbons and separate ring systems. It is shown that the strain caused by fusion with a cyclopropane is the sum of the separate rings, whereas the strain resulting from fusion with cyclopropene leads to strain energies much larger than the sum of rings, as a result of the inverted nature of the bridgehead carbon. In all ring structures but one, cyclohexane, substitution by nitrogen and phosphorus is favorable and the effect is most pronounced for the most condensed structures. The calculated strain energies correlate very well with the experimental stability and reactivity of the bicyclic iron-amino phosphirane and phosphirene complexes. © 2005 American Chemical Society.