The phase behavior of the binary system consisting of isotactic polypropylene (i-PP) and N,N',N''-tris-isopentyl-1,3,5-benzenetricarboxamide (1)-a selected member of a class of novel, versatile 'designer' nucleating/clarifying agents-was investigated over the entire additive concentration range by means of differential scanning calorimetry (DSC) and optical microscopy. Experimental phase diagrams were constructed from data obtained in melting and crystallization studies, and a simple monotectic is advanced, very similar to the previously studied binary system i-PP/1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS). In contrast to the latter, the crystallization temperature in the present system i-PP/1 was found to increase to similar to 120 T already at the lowest additive concentration employed and remained constant at further increasing additive concentration. Liquid-liquid phase separation was observed at elevated temperatures for i-PP/1 mixtures comprising more than similar to 2 wt% of 1. A study on the optical properties of the i-PP/1 system revealed that the values for haze and clarity of injection-molded plaques progressively decreased and increased, respectively, in the concentration range up to 0.15 wt%. An intermediate region of fairly concentration-independent optical properties was found between 0.15 and 1 wt%, followed by a rapid increase in haze at concentrations exceeding 2 wt%. (c) 2005 Elsevier Ltd. All rights reserved.