The transformation of Polymorphs (alpha and beta modifications) of 2,4,5,6-tetrachloro-1,3-benzenedicarbonitile (chlorothalonil) to each other was studied by DSC method. The transformation temperature for alpha to beta was determined to be 150.5 +/- 0.1 degrees C. The enthalpy, Delta H-t, and activation energy, E-a, for the transformation alpha to beta were found to be, respectively, 4.03 +/- 0.03 kj mol(-1) (significance level = 0.02) and 6.5 +/- 0.9 x 10(2) kJ mol(-1) (95% confidence limits). The rate of transformation for alpha to beta is fast, but the reverse is very slow. The X-ray powder diffraction (XRD) patterns of the two modifications were measured, and three of the strongest lines (2 theta l degrees and relative intensity (I/I-0)) for each modification are given, relatively, 25.52 (100), 14.38 (56), 27.95 (47); 26.58 (100), 14.33 (6), and 54.77 (3). (c) 2004 Elsevier B.V. All rights reserved.