Polymer nanocomposites based on PET and with an intercalated and fairly dispersed nanostructure have been obtained in the melt state. The intercalation and dispersion levels, as well as the mechanical properties, were studied by varying the chemical nature and amount of the organic modification of the clay, as well as the clay content. The intercalation level of PET into the organoclay galleries was measured by following the increase in the interlayer distance upon mixing. The surfactant content did not influence the intercalation level but an interaction between the polymeric matrix and the surfactant, through a common polar character, led to improved intercalation. The modulus increases observed, and consequently the overall dispersion, almost did not depend on either the amount or the chemical nature of the organic modification of the clay used, suggesting that the parameters leading to a high degree of intercalation differ from those which lead to a high modulus of elasticity and therefore to a high dispersion level. The obtained increases in the modulus of elasticity that reflect the dispersion level were large, attaining a 41% increase with respect to that of the matrix after a 6 wt.-% clay addition.