The intrinsic acidity of cyclopentadiene and its PH2 and AsH2 derivatives has been investigated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level of theory. The calculated intrinsic acidity of cyclopentadiene is consistent with the available experimental data. Substitution of one of the H atoms of the C(sp3)H2 group of cyclopentadiene by PH2 and AsH2 leads to a significant acidity enhancement of 44 and 34 kJ mol-1, respectively. This acidity enhancement is the result of the balance of two opposite effects: a) a decrease in the aromaticity of the anion on going from the unsubstituted to the substituted forms, b) the stabilization of the anion in the substituted derivatives, through an interaction of the π-cloud of the ring with the X–H bonding orbitals of the PH2 or AsH2 groups. This renders the interaction of the substituent with the five membered ring stronger upon deprotonation, as reflected in a strengthening of the C–P and the C–As bonds.