The optical loss change in siloxane polymer waveguides during isothermal curing conditions is studied for the wavelengths of 850 and 1310 nm. As a first step, the evolution of degree-of-cure is determined from differential scanning calorimetry and compared to optical absorption from spectroscopy. It is found that the optical loss due to absorption mechanisms is related to the extent of cure reactions in the polymer. Optical loss due to scattering mechanisms is related to local density fluctuations, and is studied using dielectric analysis. Optical loss measurements are conducted on both uncladded and cladded waveguides during isothermal cure, which results in certain optical loss trends. Based on the trends, the underlying mechanisms for the optical loss variations are proposed and a cure schedule to realize the lowest optical loss is suggested. (c) 2007 Wiley Periodicals, Inc.