A number of substituted ethyl formylacetates and ethyl formylmethylphosphonates were synthesized and their enolization studied by NMR. The ethyl halogen-or cyan)-formylacetates and formylmethylphosphonates exhibited stable trans-enolic forms in solutions. Variation of groupings bonded to the CO or PO groups affect the chelating centre basicity. With higher basicity the cis-enolic form is stabilized