An efficient synthesis of stable phosphonate ylides and phosphonate esters is described via a one-pot reaction between activated acetylenes and triphenylphosphite in the presence of sulfonamides and heterocyclic NH-acids. Single X-ray diffraction analysis and NMR studies were used in characterizing the ylides and phosphonate ester products. Dynamic NMR studies performed on a phosphonate ylide allowed the calculation of the free energy barrier for the inter-conversion between the geometrical isomers (E) and (Z).