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Thermal index changes due to nonradiative relaxation in optically pumped Co/sup 2+/-doped and V/sup n+/ doped fibers are studied experimentally by an interferometric method. In both dopants these effects are shown to be very strong and to mask any residual resonantly enhanced nonlinearity. The measured magnitude and time constants of thermal effects are well explained by a new theoretical model, which confirms its validity and usefulness. These measurements also demonstrate a new and simple method to differentiate between nonlinear and thermal phase shifts in doped fibers based on the dependence of the phase change on the pump pulsewidth. This study provides new information on the spectroscopy of these two dopants, including the percentage of absorbed power they transform into heat (/spl sim/38% for Co/sup 2+/ and 56% for V/sup n+/), and the likely presence of clusters in Co/sup 2+/-doped silica even at very low concentrations (8 wt ppm CoO).