The thermo-mechanical degradation of PET during extrusion was studied including hydrolysis and chain extension. The active agents, water, causing hydrolysis by chain scission and pyromellitic dianhydride PMDA, causing chain extension via chain branching, were added to the melt flow as pulses. They change the PET molecular weight, affecting the melt flow molecular orientation and viscosity. They were quantitatively followed in-line by a rheo-optical detector set in an instrumented slit-die, measuring synchronously pressure drop and flow birefringence. By the addition of active pulses, the melt flow birefringence comes as residence time distribution curves. Its shape is standard when the level of chain orientation increases due to chain extension or inverted when chain orientation reduces via chain scission. The effect of the extrusion melt shearing level, made by 90º kneading blocks with different lengths, were also quantified. Off-line measurements of intrinsic viscosity and its corresponding viscosity average molecular weight corroborate the in-line measurements.