The ultraviolet (uv) absorption spectra, representing transitions to all energy levels below 44 500 cm-1 of trivalent erbium (Er3+), have been analyzed for the crystal-field splitting of the multiplet manifolds 2S+1LJ of Er3+(4f11) in C2 symmetry cation sites in single-crystal cubic Er2O3 and Er3+:Y2O3. A solid solution, without a change in the local symmetry, exists between the two compounds, allowing us to identify the weaker transitions in Er3+:Y2O3 from the stronger transitions observed in the uv spectrum of Er2O3. As a result, we have identified a complete set of energy (Stark) levels for the electronic configuration up to the absorption band-edge of these crystals. A total of 134 Stark levels representing 30 multiplets with energies as high as 44 500 cm-1 have been modeled using a parameterized Hamiltonian defined to operate within the Er3+(4f11) electronic configuration. The crystal-field parameters were determined through use of a Monte Carlo method in which 14 independent crystal-field parameters, Bqk, were given random starting values and optimized using standard least-squares fitting between calculated and experimental levels. The final fitting standard deviations between 134 calculated-to-experimental Stark levels are 5.55 cm-1 (rms error 4.89 cm-1) and 5.08 cm-1 (rms error 4.47 cm-1) for Er3+ in Er2O3 and for Er3+ in Y2O3, respectively. The excellent and consistent agreement between the experimental and calculated Stark levels in both crystals, together with the predicted sets of wave functions, are important for the ongoing analyses of intensity data and magneto-optical studies on these crystals.