TPA exhibits three crystalline forms, namely, form I, II and III of which form I is found to be stable till 500 K. When form I is heated from low temperature to room temperature, it is known to undergo a continuous order-disorder transition. From their neutron diffraction studies, Fischer and co-workers determined the onset of disorder in the H positions to be around 80 K. Their results were further corroborated by temperature dependent Raman studies by Fjaer and coworkers. The temperature dependence of the disorder is examined using solid state nuclear magnetic resonance (SS-NMR). From the dipolar lineshapes they find the proton to dynamically exchange in an asymmetric double well potential. Also, the temperature dependent study of the spin-lattice relaxation rates (T 1 )