Melt processing of poly(L-lactic acid) (PLLA) can lead to both molar mass reduction and colour formation, which can alter properties critical for medical applications (e.g. biodegradation rate). In this work, ultra-violet/visible (UV-vis) spectroscopy was applied to in-line monitor the extrusion of PLLA on a co-rotating twin screw extruder. Molar mass, viscosity and in vitro biodegradation behavior of the extrudates were analyzed off-line. UV-vis spectroscopy turned out to be very sensitive to minute colour changes of the melt. For dry PLLA a clear correlation between increasing UV-vis absorption of the melt and molar mass reduction of the extrudates is found. From the dependence of molar mass reduction on processing conditions it is concluded that thermal degradation dominates. The heat input bases on direct heating and on conversion of mechanical energy. The presence of moisture in the polymer does not significantly influence the UV-vis spectra but contributes to further molar mass reduction. In vitro biodegradation of extruded dry PLLA shows that processing parameters have an influence on the biodegradation behavior. A good correlation between the biodegradation rate of the extrudates and the UV-vis absorption of the melt is found. This investigation demonstrates that UV-vis spectroscopy is powerful for real time detecting the thermal degradation of PLLA during melt extrusion. (c) 2008 Elsevier Ltd. All rights reserved.