Effects of enhanced electron-phonon interaction of RE3+ ions at the ends of 4fn series. Vibronic structure in optical spectra of Yb3+ in YAG and Y2O3 and Ce3+:YAG. New spectral data on Yb3+ spectra in Yb and (Nd,Yb)- Y2O3 doped ceramics. Broadening of electronic lines and enhanced intensity of vibronics by quasi-resonant effects. One micron spectra of Yb in YAG and Y2O3 ceramics belong exclusively to Yb3+.