Bottle grade recycled PET referred to in future as rPET-bg can replace and be used as an alternative to virgin PET and reducing environmental issues related to landfilling and incineration. rPET-bg is considered to lose its inherited properties through recycling and reprocessing cycles. During recycling, rPET-bg gets cross contaminated with other polymeric materials, especially PP. This cross contamination is a major issue in rPET-bg recycling industry because of its influence on the characteristics of the final produce. Furthermore, during reprocessing of the contaminated rPET-bg, the transparency of the final produce is altered as a result of contamination. The present study, focussed on the influence of PP contamination on the thermal characteristics of rPET-bg, especially crystallinity which dictates transparency in PET, and its importance in bottle manufacturing. Crystallization kinetics of engineering grades have been extensively studied [1] and [2]. However, studies on recycled materials such as rPET-bg have been rare. The influence of polymeric contaminants, such as PP, on the crystallization of rPET is particularly important as PP bottle tops are easily mixed with rPET-bg when recycling. Their presence in small amounts in rPET-bg can significantly affect crystallinity during multiple extrusions. Some researchers have studied the degradation of rPET during processing and concluded that chain scission can occur and that formation of grafted copolymers and crystallization can be facilitated [4] and [5] and others concluded that during repetitive extrusion, chain scission predominated and no chain branching or cross-linking were observed which decreased capabilities and crystallinity [7] and [8], whereas others concluded that cross-linking and chain branching occur during extrusion [9] and [10]. This research study investigates the influence of thermo-mechanical degradation during repetitive extrusion cycles on the crystallinity of non-contaminated and contaminated rPET matrix with 5% PP. Furthermore, the Avrami model may properly describe the behaviour of crystallization of the non-contaminated and contaminated rPET under isothermal crystallization process.