Absorption and emission spectra of Er3+-doped and Er3+-Yb3+-codoped phosphate glasses have been studied within the temperature interval from 10 to 300 K. The analysis of Er3+ Stark splitting for the excited state I-4(13/2) and the ground-state I-4(15/2) multiplets is performed. Simulation of inhomogeneous broadening of Stark levels has shown that all (2J+1) components of both multiplets are split by random distortions of the initial symmetric polyhedron, consisting of the neighboring oxygen ions in phosphate glasses. Inhomogeneous broadening of Stark levels is different for each energy level and reflects the sensitivity of a particular energy level to the variation of crystal field. At least three energy levels participate in shaping the low-energy side of the emission spectra, caused by the I-4(13/2)--> I-4(15/2) transition. The emission linewidth corresponding to the transition to the highest by energy Stark component is assumed to derive from comparable contributions of inhomogeneous and homogeneous broadening, the latter caused by intra-multiplet transitions. The presence of the highest Stark level in the crystal-field splitting of the ground-state I-4(15/2) manifold with a high rate of nonradiative relaxation is essential for writing the rate equations which describe the laser operation of erbium glass lasers.