1. The gram-molecular susceptibilities of carefully purified samples of phthalic acid and its anhydride are determined. The susceptibilities are: Phthalic acid, Xm=−81·24×10−6 Phthalic anhydride, Xm=−67·31×10−6 It is found that whereas the value of the gram-molecular susceptibility of phthalic acid found by Gray and Birse and ourselves is nearly equal, that for the anhydride is higher in our case. 2. The difference in gram-molecular susceptibilities of dibasic acids and their anhydrides is nearly equal to that of water with minor variations, possibly due to different structures attached to the anhydride rings. 3. The constitutive effect of the rings in the anhydrides of different dibasic acids has also been discussed.