A reaction of S-methyldiethylphosphinate with the (dichloromethyl)arenes was first studied to develop a new approach to the synthesis of the dimethyl dithioacetals of the arenecarbaldehydes without use of methyl mercaptan. The most probable route of this reaction was predicted proceeding from electron structure of S-alkyl esters of P(IV) acids: the values of the IP n_s are considerably lower (9.03-9.30 eV), than the values of the IP pπ,o (9.81-10.54 eV). Therefore MeS group has higher electron donating properties than P=O group. Thiol sulfur (SMe) first attacks the methine carbon atom of the (dichloromethyl)arene. One or two chlorine atoms are substituted depending on the ratio of the initial reagents, 1:1 or 2:1, resulting in the formation of chlorothioether and dimethyl dithioacetal. Thus a new approach to the synthesis of the dimethyl dithioacetals of the arenecarbaldehydes based on mono- and di(dechloromethylthioylation) of the dichloromethylarenes with S-methyl diethyl phosphinate was developed.