The comparative νCO IR analysis between β-carbonylphosphonates [XC(O)CH2 P(O)(OR)2: X=Me 1, Ph 2, OEt 3 , NEt2 4 and SEt 5; R=Et] (series I) and β-carbonylsulfones [XC(O)CH2SO2R: X=Me 6, Ph 7, OEt 8 , NEt2 9 and SEt 10; R=Et] (series II) along with ab initio 6-31G** calculations of 1a and 6a (R=Me) suggest the existence of only a single gauche conformer for I. The negative carbonyl frequency shifts for both series follow approximately the electron-affinities of the π*CO orbital of the parent compounds MeC(O)X 11-15. The less positive asymmetric sulfonyl frequency shifts (ΔνSO2) for II in relation to the phosphoryl frequency shifts (ΔνPO) for I and the larger negative carbonyl frequency shifts for II with respect to the corresponding values for I are in line with the upfield 13C NMR chemical shifts of the carbonyl carbon for II compared to I. These trends agree with the shorter O(SO2)...C(CO)¬ contact in comparison with the O(PO)...C(CO)¬ one and are discussed in terms of Olp * π *CO charge transfer and electrostatic interactions which are stronger for series II than for I, indicating that the sulfonyl oxygen atom is a better electron donor than the phosphoryl oxygen atom. Intrinsic geometrical parameters of O=S-CH2 and O=P-CH2¬ moieties seems to be responsible for this behaviour indicated by X-ray and ab initio calculations of dialkyl methylsulfonylmethanephosphonate MeSO2CH2P(O)(OR)2 (R=Et 18, Me 18a).