Semiaromatic polyamides are high-performance thermoplastics with a huge potential but also a high price. In this study, the condensation kinetics of aliphatic diamines with aromatic diacids, diesters, and polyesters were investigated, to find ways of producing semiaromatic polyamides at lower costs. The condensation of hexamethylene diamine with phthalic acids and their dimethylesters are discussed in this Part I. The amidation of phthalic acids turned out to proceed like the amidation of adipic acid leading to PA66, only more slowly. The amidation of dimethylphthalates proved to be difficult. It is accompanied by a side reaction of amino alkylation which prevents the growth of long polyamide chains. This side reaction was investigated kinetically in detail, using model compounds that were unable to polymerize. The aim was to provide a basis for attempts to amidate polyesters, in particular, PET, which will be reported in Part II.