Fictive temperature (Tf) of pure silica cores was determined for a series of polymer-clad optical fibers via reflectance FTIR spectroscopy. The core diameters of the studied fibers were in the range 125-1500 μm. The fictive temperature was found to decrease gradually with increasing core diameter, which correlates with the anticipated fiber cooling rate. For a 1500 μm fiber, the radial distribution of the fictive temperature was examined. No dependence of Tf was observed upon the distance from the fiber axis, indicating that the heat conduction inside the silica fiber was much faster than the heat convection away from its surface, even for fibers with diameters greater than 125 μm.