The existence of the keto-enol equilibrium in diluted solutions of ortho-substituted α-methylthio-α-diethoxyphosphorylacetophenones X-PhC(O)CH[SMe][P(O)(OEt) 2 ] (X=OMe 1, Me 2, F 3, Cl 4 and Br 5) has been detected through IR (ν O H , ν C O and ν P O ) and NMR ( 1 H, 1 3 C and 3 1 P) analyses. The enol form exists as a strong intramolecular hydrogen bonded 'conjugated chelate'. For compounds 1 and 3 for which the ortho-OMe and F substituents are almost coplanar with respect to the C O group, their strong +M effect gives rise to a strong π X -π P h C ( O ) conjugation, which hinders the enolization of the α-methine hydrogen atom (circa ~4% for the enol form). Similarly, the weaker π C H 3 -π P h C ( O ) hyperconjugative effect of the coplanar o-Me-PhC(O) moiety, in compound 2, is responsible for a slightly larger amount of the enol form (circa ~22%), while for 4 and 5, the Cl and Br substituents lead to deviation from planarity in relation to the C O group and only the strong -I effect plays a significant role, being responsible for the presence of larger amounts of the enol form (>50%).