The catalytic activity towards exchange reactions in poly(ethylene terephthalate) (PET)-bisphenol-A polycarbonate (PC) reactive blending was compared for various lanthanide compounds (based on europium, cerium, samarium, terbium and erbium), and for titanium- and calcium/antimony-based catalysts. The effect of reaction time on the extent of reaction was studied by selective solubility tests coupled with H-1 n.m.r. and a selective degradation procedure for PC sequences was carried out to achieve information on the PET block length change. Each of the above methods can be used to get information on the reaction extent; however, a better understanding of the reaction mechanism is achieved by using more than just a single method. Titanium showed a higher catalytic activity; however, lanthanide catalysts, especially those based on samarium, europium and cerium, allowed the block length in the PC/PET block copolymers formed by exchange reactions during melt mixing to be controlled more easily, and at the same time they did not promote the side-reactions that occurred in the presence of titanium- or calcium/antimony-based catalysts.