We have investigated the optical properties of sol–gel thin films of tin dioxide (SnO 2 ) codoped with Er 3+ –Yb 3+ as a function of Yb 3+ concentration. The Judd–Ofelt model has been applied to absorption intensities of Er 3+ (4f 11 ) transitions to establish the so-called Judd–Ofelt intensity parameters: Ω 2 , Ω 4 , Ω 6 . Various spectroscopic parameters were obtained to evaluate their dependence and the potential of the samples as a laser material in the eye-safe laser wavelength (1.53μm) as a function of Yb 3+ concentration. An amelioration of the quality factor Ω 4 /Ω 6 was found with Yb content. Both the IR photoluminescence (PL) intensity and the up-conversion emission, from Er 3+ ion in SnO 2 , were found to increase with Yb concentration. We show that the Yb 3+ ion acts as sensitizer for Er 3+ ion and contributes largely to the improvement of the spectroscopic properties of SnO 2 :Er. The mechanism of up-conversion emission is discussed and a model is proposed. The results showed that sol–gel SnO 2 is promising gain media for developing the solid-state 1.5μm optical amplifiers and tunable up-conversion lasers.