The structures of the acid-paste-prepared a-form of cobalt phthalocyanine (alpha-PcCo) and of its corresponding beta-form (beta-PcCo) have been refined through X-ray powder diffraction using the Rietveld method. The ct-polymorph is triclinic, space group P (1) over bar, cell parameters a = 12.090(8) Angstrom, b = 3.754(2) Angstrom, c=12.800(9) Angstrom alpha = 88.96(6)degrees, beta = 90.97(6)degrees, gamma = 95.09(7)degrees, and Z = 1; the beta-form is monoclinic, space group P2(1)/a, cell parameters a = 14.5982(9) Angstrom, b = 4.7937(3) Angstrom, c = 19.4348(11) Angstrom, beta = 120.782(3)degrees, and Z = 2. alpha-PcCo consists of columnarly arranged molecules, those of adjacent columns being aligned parallel to each other, whereas a nearly perpendicular arrangement is present in beta-PcCo. alpha-PcCo also differs from the alpha-forms of PcH2 and PcPt in which an angle of ca. 125 degrees is observed between molecules of closely contacting chains. From the refined peak-profile parameters of the powder pattern a crystallite size of similar to 150 A has been estimated for alpha-PcCo, a value approaching that of nanocompounds. The isothermal alpha --> beta phase transition has been followed in-situ real-time by means of EDXD at two different temperatures, 152 and 250 degrees C. Calorimetric data indicate two different behaviors of the rate constant in the temperature ranges 182.5-220 and 220-270 degrees C leading to values of the empirical activation energy, E-a, of 36.6(16) and 14.8(9) kcal/mol, respectively. However the various DSC runs lead to values of the phenomenological n parameter in the range 2.34-2.68 (indicating an isokinetic two-dimensional growth). According to the combined EDXD and DSC data a three-step model for the alpha --> beta PcCo phase transition may be proposed: (1) disordering of adjacent layers of PcCo molecules from their original alpha-type arrangement; (2) crystallization of the beta-form from the disordered phase through rearrangement of the layers of the phthalocyanine units; (3) crystal growth of the beta-phase from the nuclei to an average particle size of similar to 2500 Angstrom, as indicated by the fitted peak-profile parameters. No evidence of the occurrence of intermediate ordered phases was observed.