Various mono-substituted diethyl arylphosphonates were prepared in good yield by treatment of aryl bromides or iodides with triethyl phosphite in the presence of NiCl2or alternatively, addition of the aryl grignard reagent to diethyl phosphorochloridate. These esters were then converted into the appropriate phosphonic dichlorides by standard methods.Para-substituted phosphonic dichlorides were also prepared by dimethylsulfoxide oxidation of phosphonous dichlorides which were prepared by Friedel-Crafts reaction of PCl3with the substituted benzene. The P-31 chemical shifts of these phosphonic dichlorides were correlated with Hammett (σn) and Taft (σo) values. The shielding of the phosphorus resonance signal by electron withdrawing groups was rationalized by increased P˭O bonding. The observed shielding ofortho-substituted arylphosphonic dichlorides was rationalized by a gamma effect.