The upconversion properties of Er3+ doped and Yb3+/Er3+ codoped NaYS2 crystals are presented. Under near-IR excitation around 10 000 cm(-1) intense up conversion emission from the Er3+ F-4(7/2), S-4(3/2), F-4(9/2) and I-4(9/2) excited states is observed at 12 K. A favorable green S-4(3/2) to red F-4(9/2) photon ratio of 8 is observed in Er3+ doped NaYS2. Low lying S2- to Yb3+ ligand to metal charge transfer transitions interact with Er3+ emitting states in Yb3+/Er3+ codoped NaYS2 and essentially quench the green emission. In contrast to many oxide and halide materials, Yb3+ is not a suitable sensitizer for Er3+ UC emission in this sulfide host lattice. (c) 2005 Elsevier B.V. All rights reserved.