Abstract The [3+2] cycloaddition (32CA) reactions of norbornadiene with aryl and alkyl substituted diazomethylphosphonates (SDAPs) have been studied within the molecular electron density theory (MEDT). Electron localization function (ELF) shows the presence of a pseudoradical C3 center at the alkyl SDAPs, classifying them as pseudo(mono)radical species, while the aryl SDAP is classified as a zwitterionic one, owing to the delocalization of the C3 electron density on the C3‐P bond. These 32CA reactions present high activation enthalpies in dichloromethane, between 17.3 and 21.6 kcal mol −1 , being strongly exothermic by more than 33.8 kcal mol −1 . While these 32CA reactions present a total syn facial selectivity, the endo/exo stereoselectivity depends on the steric hindrance caused by the bulk substituents present in these SDAPs along the approach to norbornadiene. Interestingly the phenyl group provokes a lesser hindrance than that of the small methyl group, justifying the 95 % of exo stereoselectivity experimentally observed. ELF and the AIM (Atoms‐in‐Molecules) analyses account for the bonding changes along these 32CA reactions. The energy cost for the rupture of the norbornadiene C−C and that of the SDAPs N−N double bonds are the major contributors to the activation energies of these non‐polar 32CA reactions.