A study to evaluate the thermorheological properties of organic in situ composites with short polymer fibres dispersed in a thermoplastic matrix has been carried out. High-density polyethylene/Poly(butylene terephtalate) HDPE/PBT, HDPE/EVA-9/PBT and HDPE/(EVA-9-Bu2SnO)/PBT (in situ compatibilised blends) were melt blended in a twin-screw extruder, drawn at the die exit and cooled. Morphological results revealed that it is possible to generate fibriliar morphologies for high PET concentration and draw ratios higher than 1. Compatibilisation with in situ generated PBT-g-EVA-9 compatibiliser was achieved. Rheological properties in the melt state and in the solid state were used to analyse the compatibilisation and the reinforcing effect of fibrils. Two phenomenological models, Takayanagi and Halpin-Tsai, were used to describe the variations of E ' and tan delta as a function of temperature,