Electron withdrawing groups have a strong effect on hydrogen bonding to aryl ureas. The effect of para substituents modestly exceeds the effect of meta substituents. Among common substituent parameters, σ−(ϱ= 1.77, r2= 0.988) is found to be the best predictor for the observed effects of para substituents in aryl ureas. Semi-empirical and ab initio methods are used to calculate charge distributions in substituted benzene derivatives and in these ...