The transit times of light pulses as functions of temperature in nylon and acryl-coated fibers and hard-clad silica (HCS) fibers are analyzed theoretically and compared with the measured results. The effect of temperature on the transit time can be explained in terms of generally known models and the physical and thermal coefficients of the materials. The thermal effect in a core-cladding interface can have unpredictable consequences, however, as in HCS fibers. Because of the increased inhomogeneities at the interface of the core-cladding, the transit time of a light pulse increases, in contrast to that in a glass fiber, at lower temperatures. Thus the light pulses have larger angles of propagation than is shown by the numerical aperture of the fiber, and at the same time the attenuation of the fiber increases.