The formation of the trigonal structure of isotactic polypropylene (i-PP) in a series of random terpolymers of propylene (C3) with n-pentene-1 (C5) and n-hexene-1 (C6) by cooling from the molten state was investigated with the aim of assessing to what extent the intrinsic molecular disorder and the length of the side branch of the comonomers reflect on structuring kinetics and features of the crystal lattice. Experiments were performed on samples with an overall content of comonomers of ca. 25 mol% and show that, alike in the random copolymers of propylene with 1-pentene (C3/C5) or with 1-hexene (C3/C6), the trigonal modification of i-PP, form, develops upon cooling from the molten state also in the case Of random terpolymers. At a constant molar fraction of propylene, the values of a and b axes of the unit cell linearly depend on the relative proportion of the two comonomers. Crystallinity and cell constant values as a function of terpolymer composition were provided by evaluation of data obtained from deconvoluted WAXD patterns. The development of crystallinity in isothermal conditions was followed at 0 and 20 degrees C, both by WAXD and DSC. Results were compared with those of the correspondent C3/C5 and C3/C6 copolymers and used to confirm the kinetic model proposed in a previous paper. (C) 2013 Elsevier Ltd. All rights reserved.