By the resonant inverse photoemission spectroscopy ͑RIPES͒, we study the crossover from mixed-valent to heavy-fermion and Kondo regime in CeCoGe 3Ϫx Si x (0рxр3). We have measured the RIPE spectra as functions of composition and temperature. The strong composition dependence of 4 f state ͑''f 1 peak''͒ just above the Fermi level shows that the localized 4 f electrons in the compound become itinerant due to the enhanced Kondo effect within the range 1.0рxр1.5. Temperature dependence of the f 1 peak indicates a transition from the paramagnetic state at room temperature to a coherent Fermi-liquid regime at low temperatures where the Kondo effect dominates. The most remarkable variation of the spectra has been observed above 200 K with increasing temperature in CeCoSi 3 which has a very high Kondo temperature T K of about 900 K. This suggests that the change in the 4 f-electronic state of CeCoSi 3 may be scaled by the coherence transition temperature rather than T K. ͓S0163-1829͑99͒50638-3͔

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